What is the oldest manuscript writen in a Romance language?

Assuming the Romance languages descend from Vulgar Latin, how can we know if a very old manuscript was writen in Old French for example and not in Vulgar Latin or a mixture of Vulgar Latin and an early form of Romance?
In which language was the first Romance text writen, French, Spanish, Italian? Which Romance language started diverging from Latin earlier?

The oldest written French text (Anno 842) is Le Serment de Strasbourg (the Oath of Strasbourg) — which, being bilingual, was also the oldest German text (Strasburger Eide):

French Romance:
"“Pro Deo amur et pro Christian poblo et nostro commun salvament, d'ist di in avant, in quant Deus savir et podir me dunat, si salvarai eo cist meon fradre Karlo et in ajudha et in cadhuna cosa, si cum om per dreit son fradra salvar dift, in o quid il me altresi fazet, et ab Ludher nul plaid numquam prindrai, qui, meon vol, cist meon fradre Karle in damno sit.
Si Lodhwigs sagrament que son fradre Karlo jurat conservat et Karlus, meos sendra, de suo part n lostanit, si io returnar non l'int pois, ne io ne neuls cui eo returnar int pois, in nulla ajudha contra Lodhuwig nun li iu er.”

Old German:
"“In godes minna ind in thes christiânes folches ind unsêr bêdhero gehaltnissî, fon thesemo dage frammordes, sô fram sô mir got gewizci indi mahd furgibit, sô haldih thesan mînan bruodher, sôso man mit rehtu sînan bruodher scal, in thiu thaz er mig sô sama duo, indi mit Ludheren in nohheiniu thing ne gegango, the mînan willon imo ce scadhen werdhên.
Oba Karl then eid, then er sînemo bruodher Ludhuwîge gesuor, geleistit, indi Ludhuwîg mîn hêrro then er imo gesuor forbrihchit, ob ih inan es irwenden ne mag: noh ih noh thero nohhein, then ih es irwenden mag, widhar Karlo imo ce follusti ne wirdhit.”

Le texte "français" ressemble à un étrange mixte d'occitan ("pro Christian poblo et nostro commun salvament") et d'italien modernes ("altresi", "in quant", "in cadhuna cosa"), avec quelques mots latins qui paraissent artificiellement introduits pour dignifier la langue (p.ex. "numquam").

Le français moderne a considérablement évolué depuis lors, l'occitan, l'italien et l'espagnol presque pas.

C'est pourquoi la question suivante
"Which Romance language started diverging from Latin earlier?"
est sans objet.
Tout se passe comme si les langues romanes avaient divergé à partir d'un substrat commun italo-occitan!

Une brique de plus pour fonder la théorie de Cortez?...

<<<<<<<<
The oldest written French text (Anno 842) is Le Serment de Strasbourg (the Oath of Strasbourg) — which, being bilingual, was also the oldest German text (Strasburger Eide):
>>>>>>>>

The oldest Germanic texts are runes written in the runic alphabet Futhark. The earliest runic inscriptions date from 150 and the alphabet was generally replaced by the Latin alphabet with Christianization, by 700 in central Europe and by 1100 in Scandinavia.

One further old Germanic text is the Bible translation of Ulfilas or Wulfila (ca. 310 – 383). Ulfilas translated the Bible from Greek into the Gothic language. For this he created the Gothic alphabet. Fragments of his translation have survived, including the Codex Argenteus, in the University Library of Uppsala in Sweden.

So Germanic languages is by far better documented in medieval history than any Romance or vulgar Latin language. Why it is so is still unclear...

Tim, you are right.
Until then, and still for a long time, only Latin was written in Romance areas.

What makes the Oath of Strasbourg so unique is that it delivers an approximate insight about the actual vernaculars spoken in the Frankish Empire. From Wikipedia:

"In 842, Louis the German, ruler of the eastern Frankish kingdom, met with his brother, Charles the Bald, ruler of the western Frankish kingdom, at Strasbourg. At this meeting, Louis and Charles pledged their allegiance to each other -- and their opposition to the Emperor, their elder brother Lothar. According to our chief source for the meeting, Nithard[?]'s Life of Louis the Pious, each king swore the oath in the vernacular of the other's kingdom."

Actually, the bulk of the treaty was in Latin, only the final sentences were pronounced in Romance and German so that both king would be fully understood by the other side's warriors and be fully committed to them. Therefore they used languages that common foot soldiers were able to grasp.

"sôso man mit rehtu sînan bruodher scal, in thiu thaz er mig sô sama duo, indi mit Ludheren in nohheiniu thing ne gegango, the mînan willon imo ce scadhen werdhên":
all the uniqueness of modern German syntax is already there!

are there any texts in old latin?

The oldest manuscripts writen in a Romance language are Los Cartularios de Valpuesta. They were writen in primitive Spanish in 804.

"
<<<<<<<<
The oldest written French text (Anno 842) is Le Serment de Strasbourg (the Oath of Strasbourg) — which, being bilingual, was also the oldest German text (Strasburger Eide):
>>>>>>>>

The oldest Germanic texts are runes written in the runic alphabet Futhark. The earliest runic inscriptions date from 150 and the alphabet was generally replaced by the Latin alphabet with Christianization, by 700 in central Europe and by 1100 in Scandinavia.

One further old Germanic text is the Bible translation of Ulfilas or Wulfila (ca. 310 – 383). Ulfilas translated the Bible from Greek into the Gothic language. For this he created the Gothic alphabet. Fragments of his translation have survived, including the Codex Argenteus, in the University Library of Uppsala in Sweden.

So Germanic languages is by far better documented in medieval history than any Romance or vulgar Latin language. Why it is so is still unclear...
"

Germanic words and names are also often mentioned in Latin texts written by Roman historians or geographs. How can it possibly be that a (until today hypothetical) language like "proto-Romance", "proto-Italian" or "vulgar Latin" was never mentioned in the abundant Roman literature?

On the other hand I have heard that aquaeducts, which were built without doubt by Romans (or better by their slaves under Roman rule) in great numbers, are never mentioned in any written Latin text that is known to us. So the absence of written documents doesn`t mean too much.

"I have heard that aquaeducts are never mentioned in any written Latin"

Sextus Julius Frontinus, De Aquaeductu Urbis Romae

I repeat, your theory about proto-Italian is idiotic!!!

You`r right, it was a special aqueduct, the Eifel Aqueduct near Cologne, that was never mentioned in Roman literature. It was the biggest aqueduct Northern of the Alps and one of the longest aqueducts of the Roman Empire.