Once water mixable oils are dry to the touch, all soap (un-fancy term for the surfactants that make them water-soluble) has evaporated or has been converted.The result is that they are just like traditional oils at that point. Anything for traditional oils should work.
If you have thinned your water-soluble varnish with water, it will bead up on dried oil paint, water-soluble or traditional.
That said, there are conditions where varnishes will bead on dried oil paint. It has to do with surface tension (but nothing to do with whether there was a surfactant in the paint at the time the paint was applied.) This usually can be prevented by knocking back a bit of the gloss on the paint (or varnished surface), usually with sandpaper or by rubbing with pumice or other mild abrasive. Brisk rubbing with a clean rag will have the same effect.
Back in the bad old days, when house and wall paints were oil-based, you always had to sand your undercoats or first coats before applying a second... same thing applies to oil.
By the way, final varnishing should be delayed until the oil paint (again, whether water-soluble or traditional) has been exposed to air for 6 months or more. Linseed "dries" by reacting with oxygen in the air - it polymerizes. The polymerization has to be well advanced between final varnish is a good idea. Varnish prevents oxygen from reaching the paint film.
If you're using W&N Artisan, you might try a tube or two of Cobra or Lukas Berlin. There's a world of difference in consistency and working behaviors. I found Artisan to be sticky and stiff.