If it's an early production Vista HCx that has never been firmware-updated there will be a size limit on the mSD card of 2Gb. Updated devices and later production models don't have that restriction. 2Gb is plenty for any reasonable map, but some map downloads can get ridiculously large these days.
A Vista HCX in good condition is as good as any GPS out there, IMHO. However if it's been around the block a few times then there are concerns about reliability, and for something mission-critical like a PBP** it might be worth replacing it with a new Etrex 20x or 30x (you'd also have to shell out for a suitable handlebar mount) - but only because of the reliability, not because the newer model is in any way better, it isn't.
** of course you don't
need a GPS to ride PBP - the whole route is waymarked and by night there are plenty of red lights to follow.
If you do fit an mSD card then you can use the option to auto-record daily tracklogs to the card, well worth doing I think.
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