I cannot say that Sindarin would be safer, no. Our understanding of both languages is incomplete, but generally speaking our knowledge of Sindarin is more limited, and thus more speculative. That being said, I am personally better versed in Sindarin than in Quenya (having neglected my Quenya for a long while).
There may be lore-related consideration pertaining to the choice between Quenya and Sindarin. Quenya is the language of the Ñoldor in Valinor; the Vanyar have their own, closely related yet different dialect, of which we do not know much at all. The Ñoldor brought it to Middle-earth with their exile, but due to political influences it ceased to be their daily language, and became a language of lore. It later regained some prominence in Númenor, now spoken by humans, but by the 3rd Age it was purely a language of lore in Middle-earth. Meanwhile Sindarin became the spoken, daily tongue of the Ñoldor (and obviously the Sindar) in Middle-earth back in the 1st Age, and remained so later.
So from that point of view, you could figure out which language to use based on the context you want to give this name. But that is of course just one approach; meanwhile it is also perfectly acceptable to simply choose whichever language you like best, based on whatever criteria you choose.
As to Vanisil - the pronunciation is straitforward indeed. Just make sure your vowels are not diphthongs, and that you stress the first syllable, Vanisil. Appendix E in the Lord of the Rings explains the pronunciations relatively well, but I can try to explain more myself if you want.
I will note this though: the word isil and some other words that you will see written with 's' (but not all!) were originally pronounced with a þ instead (the sound of 'th' in think, [θ] in IPA), and still are by some, including loremasters; this phenomenon is expanded upon in the Shibboleth of Feanor (in The Peoples of Middle-earth). So that is another option for the pronunciation.
