You should wait for nighttime temps consistently above 50 for true tropicals (there are some plants, including certain gesneriads, that may collapse in temps below the low to mid 50s).

If properly hardened off, cannas and sweet potato vine can handle temperatures in the 40s at night.

In a sheltered, shady location with no direct or even dappled sun, the bananas should not get foliar burn. Tender vegetation that's unused to drying winds can get damaged even in shade, though.

For the bananas, try cheesecloth or remay (a floating rowcover product used by veggie growers).

Are my plants dead? I took them out too early and we had a couple of cold nights, leaves turned brown and shrivelled up. (Cannas and potato vine especially). How long would you give them a chance?

I'd give them some time, sojay, as both cannas and sweet potato vines have a tuberous root that is surprisingly hardy if the freeze wasn't too hard.