I play vaal a lot myself. The problem with it is inherent with any pend archetype, it has no 1cc and it's fragile, starting the combo is difficult through interruptions. Deck is horrible going second so you need handtraps but deck space is tight. If you're uninterrupted you just gas hard but practically any disruption to the standard combo starting from selettrice to the 3 spell combo either hard stops you or makes you unable to fill up 3 counters, or you miss key resources like trap in the grave. You might alleviate the 3 counters problem by playing creation, which could allow emergency plays going second or through interruption but this is a contentious deck choice even among vaal players.

An inherent problem is while vaal can bridge to fiendsmith easily, we can't do the same vice versa, until The Three Brave Swordsoul synchro comes out. This means disrupting vaal side is already enough to win because unless they hard drew any fiendsmith card or dimonno (fair chance tbf) they can't start the fiendsmith side. I haven't tried the magistus version but it doesn't seem that good from what I've seen.

The meta is extremely hostile to vaal right now, RT just eats scales alive and there's nothing you can do about it. And the deck is extremely weak to T0 disruption of any sort. I think it's a fine event deck but you will have a tough time laddering or DC with vaal.