Congratulations on getting the visa
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I did see however you can pay a fee to transfer it, if it's not used in time but I didn't see if there is a maximum limit on how long you can hold out until renewal, say a month or 2 after it's expired to get a replacement and is this a big hassle?.
You can pay a fee to transfer it - it's currently £169, but is reducing to £154 from April 6th when the new fees come in.
I believe though that you'll need to wait until after the vignette has expired before you apply for a vignette transfer, and the transfer involved filling out a new application form, I think you may need to do biometrics again, and then it needs to be sent off to New York for processing... which could take a few weeks (or you could pay for priority processing which is £184 at the moment, but going up to £212 on April 6th, but that's even more money)
I'm not sure if you can request a date a certain amount of time ahead, but, to be honest, by the time you actually apply for it and receive it back, a couple of months could well have passed anyway.
Bear in mind though, that the actual visa (the BRP card) will only be valid for 33 months from the date the visa was issued in March, so you'll need to make sure you enter the UK within 3 months of that date (i.e. by June), or the visa will expire before you qualify for the next one.
Rather than applying for a vignette transfer, is there any way you could borrow the money from family or friends for a flight now, or put the flight on a credit card or something, so that she can fly within the 30 days?
It could be much more expensive to fly in the summer anyway, which means even more money you need to save, so you might end up saving yourselves a couple of hundred pounds (or more) in airfare if she flies sooner rather than later. Not to mention, the cost of transferring the vignette, plus using priority processing if you chose to, plus the shipping to and from New York, would probably cover a good chunk of the airfare anyway.