1975 vette outside mirror vibrates ....won't stay in place....no screws to tighten that i can find

If I remember correctly, you mirrors are GM's "Sport" mirrors identified by the mirror mounted inside an aerodynamic housing. If so, you will need to remove the mirror (and housing) from the door to get to the inside and tighten the actual mirror on its mount.

To remove the mirrors, I believe you need to take off the door cill first. It is a chore, but it should work as "taver91" said.