On the contrast, number of VESTS is the underlining/basic data. Another basic data is VESTS:STEEM conversion rate.
The Steem Power number shown on steemit.com is a derived data, which is calculated with a formula : SP = VESTS * rate. It just has better looking.
So when you have no VESTS, the number of SP will show zero.
All those voting / curation reward calculation are based on number of VESTS.
When you have not enough VESTS, you can't vote.
When you have no VESTS, you do nothing. Yes, nothing, can't post, can't transfer..
It's in the code.
Have you read the whitepaper? Link is here: https://steem.io/SteemWhitePaper.pdf