Roy is old french (ancien) and Roi is the today's spelling.
I looked up four Old French dictionaries and all listed 'Roi', but only one had 'Roy' as an alternate spelling. Every Old French text I looked up used the spelling 'Roi'. Of course Old French is a highly inflected language, so it usually appeared in the form 'Reis' (singular nominative with the article 'li', or plural oblique with 'les') or 'Rei' (singular oblique with the article 'le', or plural nominative with 'li').
Using my limited knowledge of Old French, I believe the phrase
long live the king would be
Vive lunge li Reis, "Vivre" only having the meaning of 'to live'.