Ok - here are my two cents.
I use a syringe and needle for my Tanex use. It does take a bit of getting used to how hard to push the plunger. It does work for me.
I have experimented with a brush, but like the syringe better.
The way I use the glue is this
1. Clean the parts
2. Test fit the parts dry and make sure there is a good connection.
3. Find a good out of the way place to hold the pieces together - FAR from where the glue will end up or be applied. Potentially use a clamp.
4. Refit the parts and have the syinge ready and position the seam vertically (gravity can help draw down the glue. I know capilary action works really well too).
5. Place the needle point at one end of the seam and gently squeeze out a drop of glue. Yes only one drop at first - it may be enough. Gravity and capilary action will draw the glue down the length of seam.
Things to watch out for, the glue (tanex) is very thin and flows into any gaps, so if your fingers are near the gap glue will be drawn under it and you are now part of your model and have lots to clean up. The glue does need a well 'mated' seam. This glue does not fill very well. It is fast acting so be sure of what you apply it to.
Potentially you may want to explore other glues like Testors Model glue or super glues like Zap. You'll hear CA (I forget the technical term) used in place of the word super glue.
A basic injected plastic (sprue based) model can be built with Tanex, CA, or Testors no problem. You may (and I do) use all three depending on the application.
A resin based model will use CA or two part epoxy.
A metal kit (or PE kit) will use CA or two part epoxy.
A vacu form ---- hmm, I'd experiment on some scraps, but CA, Testors would work and maybe Tanex.