How to stir fry frozen veggies?

The trick is to not use a wok at all. A wok works by concentrating a lot of heat on the bottom, more heat than you can achieve with a home stove. "Stir frying" moves the food from the cool sides of the wok, through the intense heat at the bottom, then out.

Rather, use a wide skillet. A home stove can't generate the intense, focused heat of a restaurant stove, but it can generate a lot of heat over a wider area. Put your skillet on the burner, add some oil, get it good and hot, then add your vegetables. The intense heat will defrost them and cook them quickly.

You have to work in smallish batches, probably no more than half a bag. And if you want to stir-fry meat with the vegetables, you may need to temporarily transfer the one out to make room for the other.

As for defrosting... the idea is that you don't need to: thinly sliced vegetables will quickly go from frozen to thawed to cooked when given direct access to very hot oil, and all surface moisture will quickly evaporate. Defrosting them will just give them time to get mushy.

I'll be honest, I'm not entirely convinced that that's better than defrosting them and allowing the water to drain. But I have observed that when these vegetables come out of the bag, they look nice and clear and perky, and when they're defrosted they look kinda sad. So, maybe there's something to it.

Anyway, that's my advice: use a wide pan, over high heat, with a fair bit of oil, and work in small batches.

Pre-saute the frozen stuff as one or multiple separate batches, in whatever wok or pan you have that has the best heat transfer characteristics, and then add them warm or hot to your actual stir fry. Alternatively, par-boil or par-steam (bamboo steamers are great for that) it before adding.

Or maybe it is the quality of product that matters? may apply as well.

I see many of these (frozen) veggie mixes containing ingredients with a different cooking time. Examples: bean sprouts and carrot, or mushrooms and onions, in one mix. You will never get these right, because when one ingredient is finally edible, the other will be a mush.
(Surprisingly, this is less of an issue when using a wok, then when e.g. frying them in a little oil. ^*).

This is not a 'wok-specific' issue of course.
The only solution in these cases seems to be don't buy them.

^{* Maybe because of the extreme heat, maybe because we are used to vegetables having more 'bite' when prepared in a wok.}

I would advise against stir-frying your vegetables frozen. The key to a great stir-fry is a good sear. Your food will sear the best when it's dry and cooked quickly as hot as possible. "Wok-hei" is a Chinese phrase that means "breath of the wok," and refers to the incredible flavor in meat and vegetables seared over high heat.

Like you mentioned, excess water hurts that sear because the heat of the pan is wasted evaporating water and not cooking your vegetables. The temperature of the pan drops and you get steamed vegetables instead of stir-fried vegetables.

I recommend defrosting, draining, and/or drying your vegetables before using. I also highly recommend a cast-iron skillet if your wok isn't staying hot enough. A cast-iron skillet will maintain its heat better when you add food, which makes it perfect for stir-frying. It should only take 30 seconds to one minute to sear your vegetables and they should still have some crunch.

The Food Lab has a great write-up on maintaining high heat for your stir-fry and some tips for doing so as well. Check it out!

I can see two answers to this, and you can use both.

1. Get the oil in the wok very very hot before you add the vegetables. This is good advice for stir-fry, regardless of the frozen vegetables. The oil shout be incredibly hot, starting to bubble, before you add vegetables.
2. Thaw the vegetables before using them. If you have a plastic bowl, you can put the vegetables in it, then cover with water and microwave for a minute or two. It won't be warm, but should defrost. That way, they won't thaw in the pan and cause the water to release.

It's generally better to use fresh vegetables, but like you I'm far too lazy for that. I just wanted it said.

First of all, you should use frozen food before its expiration date, defrost and bring it to room temperature by first placing it in a bowl of warm water for at least half an hour. This will bring the food to room temperature. Then remove the it from water and rub it with dry clean cloth after that it can be used for any cooking propose like normal raw food.