I make vegetarian tacos all the time, I might saute the beans with onion or garlic, but the only spices I use are cayenne or chili powder, and cumin. "Chili powder" you buy might just contain chilies, or it might also contain oregano, cumin, and even other ingredients.

"Mexican seasoning" sounds like a very scary thing! I don't know about that taco seasoning recipe either. Chili powders in my experience, vary greatly (unless it is referring to "chile powder" of a specific type of pepper). I don't like those pre-mixed spices either, but it really depends on what type of tacos you're making. Usually you can't go wrong with cumin, cayenne, salt, and then use fresh garlic and onion. I might also get some dried arbol chiles and rehydrate them then puree them with a bit of cooking liquid and oil to mix with the spices, but I looooooove spicy food! For less of a kick, you could go the same route with chipolte peppers. It would probably make it nice and smoky.

Don't forget the coriander!

When I want a Mexican flavor, I use cumin, garlic, red chile powder, and maybe a little oregano. I also love chipotle spice! Cumin is really the key, I think.

I make a mexican seasoning mix at home for recipes like this, and I use the following...no set amounts, just whatever ratios you think work best for your taste:

Ground chile (powder) (whatever kind you like--I usually do a few different kinds for a variance in heat and flavor)
Cumin
Ground coriander
Dried Mexican oregano (if you can't find it, the regular stuff will work)
Garlic powder
Onion powder
Cayenne, for heat, if desired
Paprika (mostly for color, but if you want some added smoke, use a smoked paprika)
Freshly ground black pepper
Ground white pepper
Salt (if necessary--mostly I leave this out and salt later so I can control it better--otherwise sometimes you need more salt, but don't actually need more spice, and you're stuck--or vice versa)

Emeril's Southwest Seasoning

http://www.recipezaar.com/Emerils-Southwest-Seasoning-23537

http://www.thespicehouse.com/spices/taco-seasoning

I love this taco-seasoning. I have a 1 cup jar in my pantry. I usually add a little shake of cayenne to give it a little kick.

I want to try these out because when I make tacos, I like to just mix refried beans and black beans and fry them up in some garlic and onions and then add cumin, chili powder and some prepared salsa. It gives it a nice texture and very flavorful. Here are some ideas too for a taco party:

http://abcdsofcooking.blogspot.com/2010/02/how-to-party-with-taco.html

Add some Jalapeño juice! I always keep a jar of pickled jalapenos in the fridge (which you can buy canned- in the ethnic foods isle or make at home). I add a little of the juice to whatever meat im making my tacos out of, let it simmer and then add:

Tomato paste
Onion
garlic
Cilantro
Chili Powder
Salt
Pepper