To vote on a decision, you actually need "view decisions" to be enabled. That may be implicit for some people, but it doesn't actually make much sense: you may want to just enable "vote on decisions".

The proposed fix right now is to create a new "view decisions results" permission that would allow you to see the results tab. If you have the vote permission, then you will be able to see the form. Maybe we also need a "view own ballot" permission, along with #400020: Show the user what he voted even if he can't see other's results...

(In general, we should be more careful about words like "vote", "decisions" and "ballots" being thrown all over the place and make that more orthogonal.)