So, I'm by no means an expert. But I have seen a lot of people going down this road.
One thing that did confuse me. You said she isn't eating the pellets, but also that she's been eating seeds and pellets for about 16 days. Do you mean that you started offering her pellets 16 days ago, but she hasn't eaten them or that she started eating them but then decided she didn't like them and stopped eating the pellets?
First, it's really helpful to know that this is almost certainly going to be a process that takes time. A few days isn't going to do it unless you happen to get a bird who already eats and likes pellets.
Here are my observations and you can take what seems useful to you.
Starting with where you are:
If you really know your bird isn't touching the pellets, I'd consider continuing the vitamins until you start to see that your bird is eating at least some pellets.
Since you say your bird is picking out favored seeds and only eating those, if you can, I'd start giving the bird smaller portions of the seeds at a time. This is more realistic if you're at home enough of the time that you can periodically check the seed dish and see how she's doing. I used to give my pair a tablespoon of seeds at a time (they're littles, so that's a lot for them). I backed it down to a half tablespoon and I find that they're much more open to eating all the seeds rather than just the ones they like. I'm not at all suggesting letting them go hungry, just that they don't only get to cherry-pick the "good stuff." That won't help with the pellets, but it will at least go towards providing a more balanced diet.
Two other things that might be helpful.
One, if there's a feeding station that your bird prefers, put the pellets there. When my birds were in a cage, I put their pellets in a feeder towards the top - they liked to perch there even when they weren't eating so it made it more likely that they'd take a nibble if they got hungry and just happened to have a supply of pellets right in front of them. Again, if your bird isn't yet eating pellets at all, I'd make sure the seeds are someplace she can get them easily, but just not the "best" spot.
Two, try feeding it moist. My birds came eating pellets (at least somewhat, I wouldn't have tried to make it their only food) but it wasn't their favorite. So for a long time I ignored the suggestions to try feeding moist pellets. But, when I tried it, they really do like that better than when the pellets are hard. It's a little inconvenient, because it isn't safe to leave wet food out all day, but if you're going to be around (or someone is home who can change out their feeding dishes) it's definitely worth trying.
Also, don't assume that because your bird won't eat one pellet that they won't eat pellets at all. My birds say choosy birds choose Roudybush, and that Harrison's is pretty good, too. But, your birds may have different preferences and it may take a little time to find out which ones they like. Also, some birds like one size or consistency more than another, so you may need to experiment even with the same pellet.