When when I am at a stop the A/C is blowing thru the vents but when I take off and give it gas it stops blowing thru the vents on the dash and starts kicking the cold air thru the window defrost vents on the top of the dash. It there anyway to stop this from happening?

You have a vac issue. Check the vac ball. Lines leading from it to cruise control module, if equipped, and other areas too

Is there a way to rig the vent door open so it always blows thru the vents?

It would be easier to fix the vacuum leak. If you have cruse control the leak will also cause problems with it working.

The defrost is the 'default' setting, so to preserve the ability to defog your windows even when the vacuum is lost (if you might have the question why it would switch to defrost setting and back)

Not necessarily on the Renix engines. The main vacuum line, a 1/4" rubber hose, splits into two hoses near the reservoir, with a hose going into each end of the vacuum reservoir. The vacuum reservoir is a dual chamber design (separate chambers), with one chamber for the cruise control and the other for A/C. From there two small hard plastic vacuum lines, one from each chamber, go each to its respective function, one for A/C, one for cruise control. If the main rubber hose has a leak then yes, both A/C and cruise will be affected. But usually the case is that there's a leak in one of the hard plastic lines, and thus, only one system or the other would be affected.