I'm like you, OP. There's no way I'd be able to save money for a trip without touching it. Going to Disney this month and am now cramming $5k worth of expenses in one month because I didn't stick to a savings plan.
I like your idea of paying a reputable travel agency monthly payments. Another way to do it is to pay off stuff as you go. For instance, if you know the flights you want, pay for them. When you book your hotel, pay in full. That way, when the time comes you've paid off all major stuff.