You choose whether to sacrifice a permanent (and which one to sacrifice) while Vraska's first ability is resolving. No player may take actions between the time you choose which permanent to sacrifice and the time you do so.
2018-10-05 • wotc
Tokens that aren't a copy of something else don't have a mana cost. Anything without a mana cost normally has a mana value of 0.
2018-10-05 • wotc
If a permanent has {X} in its mana cost, X is considered to be 0.