Sweet! I love baseballl stadiums and take tours whenver I can. If there isn’t a tour available, I can always find a way in, usually through the vendor’s gates like you said. The secret is looking like you “belong” there. The only stadium I got caught at was Baltimore’s Memorial stadium, and got thrown out. But I came back later ;)
I’ve been to many HS and college B-Ball games early and just went in and walked around until the stands started filling in. Not so much pros.
In the 60s it was easy to buy a General Admission ticket to the Angels and sneak down to the filed level sometimes right behind the plate or dugouts. Now that the Angels are good, not so easy.