Will those vacuume space saver bags flatten a pillow?
Those bags aren't even necessary to do that. Any sturdy bag that can be closed well will do. Just use your vacuum cleaner hose and suck out the air. And, yes, the pillow will fluff right back up. I live in a small house and have used this storage method for years. The problem is: if on a plane, they tend to open some bags while searching your luggage, and then you may have a problem with everything fitting back in. Good luck!
Can I put down jackets in space-saver (vacuum) bags to squeeze them into the one suitecase of check-in baggage
Yes, you definitely can use vacuum bags. They work great for the bulky, air-filled items you described. The above answerer is correct that you'll have to find a way to re-vacuum for the return trip. Another option is to wear at least one of the items e.g., down jacket and give yourself more room in the suitcase. Also, I'm not sure where you're traveling to, but I would not try to pack blankets. You will probably find it less expensive and less hassle to buy this item when you arrive and leave it with your host. Happy travels!
Do space bags make your luggage weigh more?
I was wondering if the space bags make your luggage heavier cause when i pick one of the bags up the are heavyyy and im going on a trip tomorrow so i dont want to pay for overweigh bag fees