agree with ggnoob1337 and forummm. However, if you really want to keep using your VTI for your ROTH, then just wait until you have $5500 and invest it all at once for a $7.95 trade fee. That will give you an additional expense of 0.14% when you purchase (and again when you sell), which is fairly negligible.
Since the VTI is just investing in Vanguard's total market, you'll wind up with the same annual expense ratio.
Buying in $500 incriments is fairly foolish, but in $5500 chunks it would just be a blip. Your call.