Strange Modelsim simulation result (undefined).

If you put the simple assignment in your process (rather than using a variable) it should also function correctly.

If you leave it outside the process (as commented out in your code) then I believe you might get contention. The process is driving the MostSignificant vector and so is the concurrent assignment. That said it does appear that the MSA bit is not driven in the process!

Simplest solution is to always drive the entire vector from the same process i.e. simplify the code to

PROCESS (VectorOnes)

BEGIN

MostSignificant(MSA) <= VectorOnes (MSA);