I set up a little blind 15 yards from a feeder. Is this too close? Also, how quiet and scent-less will I have to be to be successful? Will it even work?

I'd say it could work if the deer get used to it. How long that takes and how careful you have to be about scent depends on how much human activity there is in the area. I'm not real careful about scent when working on feeders/blinds/trail cams and often get pics of deer shortly after leaving the spot. Of course there are those who believe that old/wise deer aren't likely to visit a spot that receives a lot of human activity. But then there's so much oil and gas stuff going on all around my spot that I don't know where an old/wise buck could go and be able to avoid all human activity. Give it a shot.

We set up ours around 20 yards and usually dont have issues. I only open up one or two windows where I expect the shot to be, and nothing that will cross ventilate. Wear black to blend in with the background and use a mask or paint or something to cover your face.

It also depends on how long the blind has been up. I usually leave the windows open, they seem more skittish when something has changed. And you still have to move sloow...

good luck out there

For mature bucks you are probably going to be disappointed, and you could include mature does in this group as well.

You can fool some young deer that are curious and stupid, and could luck out with a young doe in heat having a buck throw all caution to the wind following her.

Most of the time you need to put the blind out 20-30 yards away, down wind and they have to get used to it being out year round. If it's a pop up I would say if you brush it in real good about 3 months ahead of time you give yourself a pretty good chance that a mature buck will be use to it by then.

We set all of our stands with feeders a minimum of 30 yards.

Got to be real still. I cant do it anymore. Too old! Need cover and 2o yds minimium.

I prefer to be 20-30 yards away and they will never be fooled no matter how well you brush it in IMO. Gotta give em time to get used to it.

I've set as close as eight yards away from my feeder and out to 60 plus yards. I let the deer tell me what they like and don't like. Sometimes I hunt off of telephone pole lines with zero cover and the deer aren't worried about me one bit. At other times I've placed my stand 25 feet off the ground in a thick oak tree and the deer busted me everytime. Deer can be funny animals to hunt at times.

Depends on your deer. If they like to circle the feeder before coming in, then probably yes. If they come straight in, then maybe not.
I've never used my pop up over a feeder. But where I have used it, I've seen a BIG BUCK walk in front of it at 15 yds. That day, I was in my lock-on about 100 yds away and all I could do was watch, since I was on a bow hunt. And I had just set it up the day before and without brushing it in. Just backed it up against a plum thicket. On another hunt, had a buck make a rub on a small cedar tree that was 2ft in front of my blind. Only had it up about 2 days.
Just depends on the deer where you hunt.

Sqiggy is right in some deer care and some don't. The ET deer I go after don't like anything unnatural and that includes feeders and pop-ups. Depends on area and hunting pressure IMHO

I have killed 3 bucks that were 18 yards from my stands. One was out of a 6' tripod backed up into a mesquite on a LF ranch in South Texas. He was 7 yr old buck that was within 12 yards that morning but could not get the shot with all the other deer in the area. This setup did not allow the deer to come in from the west of me at all due to the thick Sacahuista Grass and mesquite in the creek in that area. It was a really good setup. The feed pen was 18 yards away and the shot to the feeder was 25 yards. I saw plenty of bucks and does of all ages there. Seen several bigger bucks but they were not old enough for this ranch. My 2 other bucks were killed out of a boat seat I put in a Live Oak tree overlooking the feed pen. It was 18 yards to the feeder. The deer very rarely came in from the NW area and this made it a very good SE wind blind. One buck was a 3-4 yr old and the other was a 5-6 yr old. Both were shot standing under the feeder and were shot 7 days apart. My bow blind now is a haybale blind that is back into a corner of a water lot. The only wind I can not hunt is a NW or West wind which I very rarely get. I have had as many as 14 bucks within 30 yards of me at one time. I can move around and not be seen due to the darkness inside the blind. Only thing I have to watch is the noise. Some deer get within 10 yards or so if put corn to close. I killed a mature buck with my crossbow from it last year with a 25 yard shot.
Like others have posted it will depend on the deer and what you have for cover around you and behind you. I always try to protect the side of me the wind is blowing to so the deer will not approach that direction if at all possible. I always put the blind in place then feed the area with corn for several days before hunting the location to allow deer to get used to the changes put in their home.

mine are all within 20yrds... its all about staying in the shadows and working the wind