For vertical jigging what is the best weight to use?

That depends a little on the current conditions, your skill as an angler and the type of line you want to use.

The general rule of thumb is to use as light a jig as possible (so a walleye can suck it in easily) but heavy enough to feel the bottom. So, in general I would say 1/4 and 3/8 ounce jigs are my bread and butter for those depths.

Now, if you are in very fast current than you might have to move up to 1/2 ounce – if the fish are biting real lethargically and you are in slow current, maybe a 3/16 or 1/8 can be used.

One thing that will help you to maintain bottom contact with a light jig is your line. Use a thin diameter, no stretch line like Berkley FireLine. I use 6lb test FireLine in either Flame Green or Crystal. Great stuff, you will be surprised how much more you will feel over monofilament.