When I look back in my logs, my first Halloween party happened at the end of Week 4. Brenda demanding Kelly's panties happened in Week 6. My guess is you just need to relax and let the game do its thing for a bit.

Have you finished the beach party? After this you can call Ash for another beach scene and go there next day. This is key-scene for triggering Viki´s shit. I know all three early viki looking pictures blocking Brenda stealing the pants. Also the random gloryhole trigger is blocking Brenda. Can`t say if missing beach scene blocking brenda complete.