María está vistiéndose (María is dressing up)
María está vistiendo (María is wearing)
Is this meaning correct?
In María está vistiéndose the verb is pronominal, vestirse, meaning "to dress oneself". As explained in the other answer, that's a complete grammatical sentence. The verb is reflexive, that is, the object is the same as the subject.
In María está vistiendo the sentence is ungrammatical, since the verb vestir is always transitive. You're missing the object, which can be either the thing being worn or the person being dressed by María:
The meaning of the verb is not exactly the same in these two: María está vistiendo una blusa just means María is wearing a blouse (i. e. she has a blouse on), so you get a snapshot, a state, not an ongoing action. She's not putting the blouse on right now. Therefore you can sometimes say simply María viste una blusa. On the other hand in María está vistiendo a los niños it is clear that María is dressing the kids up right now, or (depending on the context) she's been dressing them up in a certain way every day recently, so the progressive tense is needed.
If you're using the transitive verb, the only way to include both the thing being worn and the person being dressed in the sentence is to turn the thing being worn into a complement:
The verbs calzar and poner can both be pronominal/reflexive or transitive and can be used in similar, though not identical, ways.
"María está vistiéndose" means that she is dressing up herself. The action "vestir" falls back over herself. It is a complete sentence.
In the other hand "María está vistiendo" seems to be an uncomplete sentence. You have to add some extra information like:
"María está vistiendo un vestido rojo" means that she is wearing a red dress.
"María está vistiendo a los niños" that means she is dressing up the kids.
I think the sifix "se" means that the verb falls back over the one who is doing the action.
"laven" means "you guys wash... something"
"lavense" means "you guys wash yourselves"
María está vistiéndose (María is dressing up) -> Indica una acción sobre uno mismo. María se viste, a si misma. Es una frase completa, con significado.
María está vistiendo (María is wearing) -> Indica una acción sobre uno mismo, o sobre otros, incluso sobre animales u objetos. María viste a alguien o algo*. Es el comienzo de la frase, falta el "quien" esta siendo vestido. Ejemplos: