when i was on a tourist visa they just swiped and i never had to sign the credit card slip. but that doesn't mean its safe for you to use someone else's credit card. i'm sure they would check the name on the card. why dont you get a visa debit card from your bank. this is just as good as a credit card. (not visa electron though). i dont know about the voucher system. i know they have it in other countries around here but not sure about doha. there's not much in the arrivals hall before you get to the immigration counter.