I have seen an 800A with the 19" wheel and fatter tire from a Vulcan 500, not sure what years, think it had a 110/90-19 tire though, would be right at same overall height as stock. The 90/90-21 should make the speedo more accurate as they read fast to start with. stock 21" rim was 1.85" wide vs the 19" at 2.15, so the 110/90 should be good.
However, the 21" is same as used on a lot of Harleys, like a 2001 Dyna. MH21 tire, most cross ref charts say it's same as 80/90, don't know why they would pick on yours. (except that it looks better than theirs?)
p.s., I had an 800A for a number of years. friend also had one, he put well over 100k miles on his. I traded for an 04 Nomad. Still miss that 800A.