[Hot, crispy nice hobbit]

I agree that Sauron is treacherous and megalomaniac, but can he be convinced that the Valar would not invade Middle-Earth again? Consider that in the Second Age, Sauron did not have sufficient strength to conquer every single race. The Numenoreans came to the aid of the Elves and, together with allied dwarves, drove Sauron to a corner. And Numenoreans are just Men, albeit beefed up with Wisdom and Knowledge. In the Third Age, Sauron had even more enemies to contend with: the Istari.

The Valar send the Istari (Maia) because they felt that a Maia should be dealt with be other Maia. But if Sauron obtained the Ring, he would have dominated or destroyed all of the Istari. By that time, I am sure that the Valar would have found him too much of a nuisance, and would just hog-tie him and throw him into space to accompany his boss.

Besides, depending on which version you read, Morgoth's will was said to be able to creep back to Middle-Earth to cause some mischief. Personally, I am of the opinion that Morgoth's Will/Spirit/Psychic power had secretly urged Sauron's subconscious to muster the Evil powers and try to release him. Consider: when Sauron was a captive on Numenor, he led the Numenoreans to worship Morgoth, even though he could have easily distanced himself with the 'jailbird of Mandos'. (And thus setup a cult to worship only himself)

No, I feel that sooner or later, Sauron would have released Morgoth from the void, if not for loyalty, than for fear of the Valar. Sauron have seem the destruction in the First Age, and he would probably have known that he would be cast out ultimately if he do not find someone who can contest with the Valar. (And even Morgoth is a poor choice, considering that he is only one against so many of the Valar)