I recommend a search on "refare" and/or "reprice," as this has been discussed many times in those threads. The basic answer: it can be anywhere from a week to never. On average, I'd say 4-6 weeks, so give it another 2 weeks before worrying. Even so, there is no reason to worry - after 2 more weeks (6 weeks total), just call Refunds and explain that the vouchers never arrived. They will void the originals and re-issue you new ones. When the vouchers are issued manually as in that case, they tend to actually arrive... eventually.

And yes, you can refare up to 3 times online. If you're doing it over the phone or in person, there is no limit to how many times you can refare, subject to the CSR being able to actually find the lower price.