When to use Seq or Vec in Chisel?

Use Seq if you just need a Scala array or container. Use Vec if you want a multiplexer.

scala.collection.immutable.Seq is purely a Scala-land concept. You can index into it via a Int thanks to its apply function.

chisel3.Vec is a hardware container that can be indexed by Scala-land Ints and hardware UInts. It has two apply overloads: apply(idx: Int): T and apply(p: UInt): T. It also have connection operators like :=, allowing for element-wise connection.

Always refer to the (scarce) Chisel documentation when in doubt. If the API in question is not documented (which is very likely), read the source code for its exact behavior.