i want to us weed killer on my vegetable garden

Roundup which is Glyphosate based is the best as it doesnt stay in the soil for months affecting future crops.

If there are already weeds on your plot then you should weedkill now.
If its currently just bare soil, then weedkilling would be a waste of time . Wait till any weed shoots appear and then kill but leave untill the weeds are about 2 inches high for the weedkiller to be most effective

It may need doing a few times every 2 or 3 weeks if you have bindweed or other perennial weeds come through

Its best to wait 2 or 3 weeks or longer. They say you can sow or plant when its dry as in same day, but best to leave it a bit longer.

After you have weedkilled a few times, sow a green manure to prevent more weeds growing. Weedkiller is less if at all effective in winter.

weed by hand ....it's good exercise :-)