Vuelling ticket purchase

I would purchase in the original currency (euros) so your card issuer (bank) does the conversion (and just the one conversion). That's my take at least.

There is no conversion fee. Depending on your card there may be a foreign transaction fee

If a transaction is processed abroad, and if your credit card charges for foreign transactions, you will pay fees. This is regardless of which website you use, and regardless of which currency your transaction is processed in. My brother bought Anne Frank House tickets a few years ago, and had difficulty because he had only warned his card about travel to the Netherlands; at least at that time, that website processed the transaction in France!

You should check the price in USD against the price in local currency; they should be close, but if they're not, definitely buy in the cheaper currency. The only other fees will be the ones assessed by your credit card; as I said, if these are assessed, it's based on the country where the transaction is processed, not the currency.

Thank you everyone for your prompt responses. I appreciate your thoughtfulness to share your experiences. I have certainly learned a few tips that are definitely not found in any guidebooks.