The vouchers available in the VIP rewards centre are linked to one's country - that is, the country linked to one's VIP account.
Note that this is not the same as the country of one's account, even though both are linked, and can't be changed online at the moment but only via the Customer Service. There seem to be some kind of redundancy in the way LEGO handles countries and languages at the moment, hopefully they'll fix it at some point. Summary:
- LEGO account (or LEGO.com profile) has a country, which can be changed online (in "Edit Profile" -> "My Information")
- VIP account has a language preference which can be changed online, and a country which cannot
In any case, when facing trouble with anything related to the LEGO website, online shop or VIP centre, it is best to contact the Customer Service directly and they'll do their best to help (and even toss in some extra VIP points for your trouble, which is a rather nice gesture).
At the moment, it doesn't seem possible to get vouchers in another currency except through changing the country linked to the VIP account via the customer Service. This means if you want to redeem VIP points for LEGO purchases, you're in effect limited to do it in your own currency (assuming the vouchers aren't country-specific; haven't tried that yet); but that's probably a minor issue for the majority of people.
Also, shopping in LEGO stores in other countries will get you VIP points without problem - the only subtlety might be that due to exchange rate, you might get more or less points for a similar purchase. As an example, I bought 21319 (Central Perk) in The LEGO House for 649DKK and got 649 VIP points; if I compare that to other countries based on the online shop I get:
- Denmark: 649DKK (~87€), 649 points, so 1 point per 0,1338 EUR spent
- UK: 64,99 GBP (~75€), 520 points, so 1 point per 0,1446 EUR spent
- Belgium: 69,99€, 525 points, so 1 point per 0,1333 EUR spent
- France: 59,99€, 450 points, so 1 point per 0,1333 EUR spent
So the VIP points given when buying a set are tied to the DKK price of that set, and seem to be aligned on exchange rates, with possible the exception of the UK (the online shop notes explicitly that VIP points are calculated at checkout). The difference in VIP points received is probably not worth taking into account when purchasing; however, the price of the set can vary a lot from country to country, and that's worth taking into account.