Here's the reply from Vultr support today:

Hello,

Thank you for your interest in our $2.50 'sandbox' plan. We are officially SOLD OUT of this extremely popular introductory offering. Please consider other VC2 sizes.

Why is this awesome plan sold out? This boils down to supply and demand. In order to maintain the high performance you've come to know and love at Vultr we limit the number of sandbox plans on each physical host node. This is to ensure the absolute best performance for our production-ready instances.

Will this inventory be replenished?

Yes. In the next 2-3 weeks we will bring additional sandbox capacity online. Please consider visiting/refreshing https://my.vultr.com/deploy periodically as it is possible this could happen sooner.

Thanks,

Joshua Brand
Systems Administrator