Question about Volo's Guide to Monsters

If i purchase the Roll 20 version will the Roll 20 character sheet allow me to use the character races in the book for "example AAsimar" or will i have to create a custom character for the sheet?

If you mean the Charactermancer (the thing in the OGL character sheet that creates a character step by step), it only supports the PHB races right now. So buying Volo's wouldn't add anything to the charactermancer just yet. It is planned to come someday though so buying for the future is something you may want to consider, plus you get all the sweet monsters. You can still use the sheet and input a Volo's race manually if you have the stats.

Seconding that the Roll20 team will be adding support for Charactermancer across the breadth of D&D5E books. For now they need to be input manually, but it is coming :)