Hello there... you don't have to worry about Vests. Steem Power is what you're looking at for voting power. Vests are a measure for those who worry about their stratification on the fish scale.
On the other hand, you can buy steem with sbd by clicking buy on steem then it opens blocktrades. In blocktrades you put steem backed dollars and the amount you wish to convert, like 1, then you select steem power on the other end and click get deposit address.
Once you do that you get a name and a memo. Go back to your wallet and click trasnfer on the sbds and then select that amount, like 1, and put the name blocktrades on the account and paste the memo from the page.
And it's done
Witnesses are like the miners of steemit, you can vote on witnesses you think will do a good job verifying transactions and etc.
Cheeers.