Not sure what you’re asking exactly. Different blending modes are appropriate for different specific situations. Mix is fine if you have a target mid-range weight that you’re trying to draw various vertices toward, as it will subtract or add as necessary according to the strength of the brush and the existing vertex weight. But if you want to get a region all to 100% then add might work better.
As for locking off vertex groups, that’s an unclear way of wording it. You could mean one of two things: either you want to lock off certain vertices to make sure you don’t accidentally add them to your active vertex group. If that’s the case you can use face selection masking in Weight Paint mode to lock off what you don’t want to touch. Or you could mean you don’t want to alter other vertex groups other than the active one.
As long as you don’t have Auto-Normalize activated you can change only the active group. This is the default anyway, but be warned that you could have unintended results. Vertices can have more than 100% total weight from multiple groups, so a vertex weighted 100% in one group and 100% in another will behave as if it is 50% in both. That’s one of the things normalize is intended to address (though sometimes you will want greater than 100% weight).
Are you complaining that no one responded to your post quickly enough? Dude, it’s Saturday.