My base coat is Tamiya acrylics and I sprayed Future prior to putting on the decals. I know the normal next step would be to seal the decals with a second coat of Future, but would Vallejo's Satin Varnish work too? Just want to avoid an unexpected reaction with the underlying Future coat.

Bill,
I have not tried to apply over future before so I can not speak to this directly. I would suggest to spray future on the bottom of the model and then Vallejo Satin Varnish as a trial.
You can spray Vallejo Satin Varnish instead of future for an excelent satin base for future weathering.