Using Vallejo water based plastic putty

No, you can sand vallejo putty, I do it all the time.  You just have to make sure it has dried and cured.

In my experience it works best for filling fine joins. An example would be along a wing root. I reach for something else for larger jobs.

I seem to recall reading that it's not designed to be sanded. I think that this is true. The putty peeled up on me the few times that I tried to. Feathering is best accomplished before the material sets. I do this by pulling a dampened swab over the bead and until it smooths out. 

It worked well for me when used within its scope of things. Just my opinion.

It is my preferred putty to use.  I like the narrow applicator that comes with it as it allows better control than the applicator on Perfert Plastic Putty.  As you know it cleans up easily with a damp or wet Q-tip, depending on how much clean up there is.  Sometimes it will take several applications to fill in a gap or hole and I have had no problems with it not sticking. 

I have never used Tamiya putty but I have used Squadron putty, which has it uses.