Agree that a diplexer will work, but there are some things you need to consider. First, everytime you go through any splitter, there is a minimum of 3db loss (Half the signal strength). This may be OK for the Dish signal because the S/N is usually significantly higher than what is required for the Dish receiver...but the OTA signal may not be as strong, unless you are getting signals above the 75% range. If they are in the50-60% range, you will "probably" not be able to get these stations.
Having said that, since you apparently can't or do not wish to run a second coax, you can not simply put a pre-amp on the OTA antenna, because most of them require a DC path to power them up, and MOST splitters do not pass the DC. There is a work around, but it does require a little patience and knowledge. Best solution if you find the signal to be weak is to use an antenna with a higher gain. I do not know what kind of OTA antenna you are using, but I suspect it is a simple one attached to the VOOM dish. If that is the case (and if you can accept a bigger antenna), you will see significant improvements with a high gain directional antenns.
This general information may not apply to your needs. It might help others that are considering the same thing. I have the same situation with one of my HiDef monitors being fed with an 811 and the other TV with a built in tuner being fed with a 501 and an external antenna directly to the TV. That way, when I get tired of paying for the HD fees and want to receive only the locals in HD, I will migrate to a less convenient method (no guide and having to use the TV remote to change from video to antenna) of receiving the signal.
