Almost all vegetarian cool water fish are bottom or near bottom dwellers, some of the catostomus species are large and primarily vegatarian (algae feeders), same with the Carpiodes like the highfin quillback (carpiodes velifer). As for non-native, you run into the same problem, most are either bottom dwellers or warm water, not both, otherwise the mrigal (Cirrinhus cirrhossus) might answer your needs. Also remember that even the most dedicated vegetarian fish will often take, mollosks, crustaceans, inscects, and even small fish, if they are available.
I can think of three exceptions that would work that are not typical. the firt lives down to around 42 F but is to large, 6'8", the Megalobrama ambycephala.
The second is the grass carp, many states require it to be triploid and so sterile, it can survive to freezing (pic by Lovshin) it gets to about 5' long, the Ctenopharyngodon idella.
And finally, the Crucian carp, the Carassius carassius (pic by CAFS) it survives down to about 36F and is a bit smaller, to 26". all have a long history of aquaculture. All are grass eaters by preference but will take water plants as well.
