Do I need to harden off my vegetable plants?

Yes, even though the soil is warm, the plants will go into shock from being indoors then being exposed to sun, wind and rain, hardening takes about 7 days, best to begin when night time temps stay above 50°, place in a sheltered location so they are protected from the elements and keep them outside, (cover with a bed sheet if you get a night time cold snap) start with 2 hours of sun, increase exposure to sunlight a few additional hours every couple of days, after a week and night time temps stay above 60° transplant warm-season crops such as tomatoes, peppers, squash, melons and cucumbers to your garden, hardy plants, i.e. onions, broccoli, cabbage and lettuce can be planted when night temps stay above 45°.

You should allow them to sit outside for at least a week before planting. Start in the shade and move into the sun over the week. The wind and sun will prepare them for outside planting.