So my group ran through The Shadow Vault today, and it was a close one. I was a little confused about the special rule that was listed and how it affected reinforcements. The Special Rule says that the Monster group that is on the River (which for our group was one Master Merriod), couldn't activate until one hero entered the Caravan tile. The third Monster group doesn't start on the map, but can enter as reinforcements, from what I gathered (otherwise what would be the point). In this case, I used goblins. My question is, because I was a little confused, is can they start spawning before the River group's condition for activation is met, or do they not spawn, or do they spawn and just move until they reach that space and then just kind of gather there in a massive mob.

I assumed, after we finished, that they can start spawning right away and start making their way to the heroes. What is everyone else's experience with this?

The only specific limitation is on the river tile monsters. Your reinforcements can continue through and around like normal. In your case the Merriod would stay in its position until a hero stepped onto the caravan - all other monsters, including the Lt can move as normal.