You can cook tri tip from frozen, just add half an hour to the cook time. You will need to reseal your bag if using freezer bags. I cook frozen tri tips 2-2.5 hours. I've tried longer cooking times but don't think they do much for such a lean cut of meat (it won't hurt though). I normally season with just salt and pepper, then make a good salsa, but you could add chili power, cumin, garlic powder, and/or oregano during the bath if you want. Or fry up half inch cubes of already-cooked tri tip with a small amount of oil and these same spices, just long enough to warm it up.

For taco meat, I like the texture at the high end of medium-rare up to medium, so 135-140°F.