Vic, to the best of my knowledge, there is no "secret" button. Vegas defaults to the last used template which is why you're seeing that particular one.
That's what I thought (after using VPro for nearly 10 years).
Nope - I set it to what I want and it defaults back to what it wants (Architect template).
Naturally, I forget - make the render and then yell and scream and throw things.
Gotta be a way without reinstalling and losing the various things that I like.
Try rendering a few seconds of what you want and let it go to completion. I think Vegas returns to the last template that actually completed a render, not the last template that was last selected.
Vegas 11 &12 seemed to me to less helpful than previous versions when it comes to selecting the render template. Seems to be heavily biased towards the casual user who just want to put something on YouTube. I had to fight my way through those preferred options to create the template I wanted. Setting it up as a custom (or one of your customs) is the way to avoid having to do it every time.
I prefer the new advanced Filtering System that VP11 and 12 have. The complexity of my templates and re-adjusts to delivery (clients, yah gotta luv 'em!), has made my experimentation with templates a valuable and valuing experience.
If a person is needing to render to a max of 2 or 3 Templates only, then this type of sophistication of filtering and adaptability is most likely over-kill.