How do you trim down a project length?
Lets say all my audio events end at five minutes, but my project is 90 minutes long because of other stuff that used to be there and is now deleted
Do you mean that you can still scroll all the way to 90 minutes? Close the file and reopen it. This will reset Vegas' end of timeline to match the project.
If clicking the "move to end of project" button still positions the cursor that far out then you still have an event on the timeline out there. It could be a single frame/sample sliver left over from trimming. highlight a section of the timeline surrounding that spot and press delete to get rid of that event.