I talked to the Master Gardeners at the Farmers' Market last weekend, and they said to expect to lose some/all of my vines eventually unless I want to spray poison (which I don't), and that if I wanted squash, I should start more seeds at intervals through June. Not exactly what I wanted to hear, but probably the bitter truth.
That said, I ignored their advice and am hovering anxiously over my plants inspecting them everytime I walk by. So far so good, and I just had my first two harvests. Still I'm walking on eggshells.
I do have a great solution for the powdery mildew that appeared on them last week after all the rain. 1/2 tsp. baking soda and one Tablespoon olive oil dissolved in a gallon of water and sprayed on each leaf. Cleared the mildew right up. Tried it preemptively on my bee balm too.