When you play a spell exiled this way, Voltaic Visionary will transform before that spell resolves.

If Voltaic Visionary transforms into Volt-Charged Berserker after it has been declared as a blocker, it will not be removed from combat and is still blocking.

Playing a card this way follows all the normal timing rules for that card. For example, if you play a land this way, you may do so only during your main phase while the stack is empty and only if you haven't yet played a land (unless another effect allows you to play additional lands).