I've not owned an inkjet for years, but I seem to recall that some paper is better than others for printing crispness and colour - if vinyl stickers aren't giving you a good enough result, you could try just using regular photo paper that you already know works. Once it's printed, there's a couple of options to turn it into a sticker.
- The expensive-and-easy option is to get a Xyron machine or similar - this will allow you to turn any printed material into a sticker.
- The cheap-and-more-difficult option is to get a regular full-page sticker and discard (or use!) the sticker part, then spray-glue the back of your printed sheet and lay down onto the sticker-paper's backing sheet. The spray-glue will not-stick to the backing sheet in a similar way to the original sticker, and then you can peel the backing off and apply the sheet like a sticker. (Or just go from spray glue to sticking it down without the backing, but stickers are generally easier to stick with the backing still part in place!)