You haven't told us exactly what equipment you've got, so I'll make some guesses here.  I think you're describing a Comcast DTA which uses the UEI Delta remote. If it looks like this one, then it can't be modified to control your Vizio.  If instead it looks like this DVR remote, we can tell you how to assign the HDMI codes to the TV button.  BTW, this latter remote is typically available new, shipped, from eBay for $6, so this can be a very cheap way to control everything within IR range.

I don't know of any Vizio remotes which can shoot the XMP IR protocol which is used by the DTA.   Other universals which will work include the OneForAll OARUSB04G (this one is nearly future proof, with support both from OneForAll and by using the RMIR freeware, in case you change or add components), the OARC05G, the RCA RCRP05BR, or the Inteset 422.

If you choose one of these, we can help you get it fully set up to control the Comcast box and the Vizio TV.  They can control the DirecTV also, but only if the remote is in the same room.