Is it necessary to add vegetables to slow cooker pot roast?
If you're doing it as a pot roast, the vegetables are mainly there to deliver flavour (unlike in a stew, where they're a major part of the dish). Sometimes they're eaten, sometimes discarded (which seems like a waste to me, so I would choose vegetables I'd want to eat).  They take on a role similar to stock or a flavoured rub or oil when roasting in an oven. 

On that basis you can generally omit them. One thing to watch out for though is that the vegetables can be used to lift the joint slightly, so it doesn't sit in a pool of fat*. In that case you'd want to replace them with something inert, or use them but discard them. 

If you then find that the meat wants a little more flavour, you can always add a little of a suitable herb and some black peppercorns, to be discarded. 



* Note that the term "pot roast" is used broadly, sometimes adding significant amounts of liquid  and sometimes not. I don't know if this is a regional thing, but my answer is meant to cover both types.