If you're talking about the $2.50 VPS, yes, that's IPV6-only, so it doesn't support IPV4. You're correct that people who only use IPV4 won't be able to access that VPS, at least not without jumping through hoops. There's also a $3.50 VPS instance with the same spec that appears to support both IPV4 & IPV6.

I've been using Vultr for a while and am quite happy with them. I've also used a couple of the IPV6-only VPSs for experiments, but I have IPV4 & IPV6 connectivity at home.