I know Vallejo makes a satin acrylic clear top coat. Besides this, what are good alternatives? I've googled and most satin top coats are lacquer. Since I use acrylic paints, I don't want to take a chance with a lacquer top coat.

I'm building a few late Vietnam-era Navy planes. The top coat needs to be semi-gloss.

The best satin varnish that I have used to date is Model-Flex from Badger.
I have used many with mixed results. This one sprayed straight from the bottle and did not clot in the brush or do any odd things on the model surface, it works perfectly.

Will you be using an airbrush?For whatever reason,I did not have very good luck with the Vallejo satin clear.I just could not get it to spray very well at all.
I have had great results using Liquitex Satin varnish#8208.It's cheap,comes in a large bottle,and should be available at any good artists supply store.I also use their gloss and matte varnish with the same results.Jeff T.