The largest survey about choice of companion animals in the United States is the APPA National Pet Owners Survey and this survey doesn't appear to ask whether people identify as vegan. However, it costs $3500 to obtain a copy of the document so I can't conclusively confirm that.
I was not able to locate any other research surveys or polls about which companion animals are most likely to be chosen by people identifying as vegan. Perhaps in the future we'll see a study from Faunalytics (previously the Humane Research Council) or even from The Vegan Society UK.
However, I think that because many vegans are likely to face restrictions about pet ownership (federal and municipal law, or restrictions on tenancy) it seems that companion animal choice among vegans is likely to closely reflect that of society overall. Perhaps an easier question to answer would be whether vegans are more or less likely to live with companion animals.