Yes, in her very first appearance in "A Good Man Goes to War", Madame Vastra uses her tongue to disable an enemy soldier:
Vastra: Was I being insensitive again, dear? I don't know why you put up with me.
(She turns and lashes Lucas with her extendible tongue.)
Doctor Who Series 6 Episode 7: "A Good Man Goes to War"
As to why she doesn't use her tongue more often, we don't have an in-universe explanation1. One reasonable possibility is that it becomes very obvious that she doesn't fit in; in "Deep Breath", we learn that this is rather important to her:
Vastra: I wear a veil as [The Doctor] wore a face for the same reason.
Clara: What reason?
Vastra: The oldest reason there is for anything. To be accepted.
Doctor Who Series 8 Episode 1: "Deep Breath"
1 Out-of-universe, Moffat probably doesn't want to waste budget on the CGI when the sword serves perfectly well