CO's ETCs are like cash, they're a form of payment, and you continue to accrue miles.

While there are always nuances, generally speaking, "dollar-off" type vouchers don't impact mileage accrual, whereas "free ticket" type vouchers will negate accrual. I'm not sure what your America West ticket was, but it sounds like it was a free RT ticket.

btw, you can even redeem that CO voucher for NW flights. Only restriction is it has to be codeshare (CO coded, NW operated), and issued by CO. It may cost you a phone call, but if you want to fly on NW on that voucher, it can be done.