Hi

If you already have a Vetta, with the HD500 you can use the stereo power amp section to plug in the L & R outputs of your HD500, or you can use it using the 4 (or 5 as it's a stereo amp in this case) cable method - with a twist.  And that twist is that you don't have to use the effects loop effect in your presets in the HD500.  That means that some presets could follow this signal path:

Guitar > HD500 Input

HD500 Left FX Send > Vetta Front Input

Vetta FX Send > HD500 Left FX Return

HD500 Left Output > Vetta Left FX Return and HD500 Right Output > Vetta Right FX Return

So what does that give you?

As long as the HD500 preset you're using has the FX loop included you can effectively place some FX like distorions so they go into the front of your Vetta and then you can use the Vetta's included FX too, and then the HD500's clean FX like reverb, delay and modulation can be placed in the Vetta's FX loop.  You'd probably want to turn off amp and cab modelling for any HD500 presets you want routed this way and let the Vetta's modelling handle this.   So this is is what's called the 4 Cable Method (with a fifth cable becase we have a stereo power amp in the Vetta).  And it doesn't stop there - you can use MIDI Program Change messages to switch patches on the Vetta to line up with your requirements on the HD500.

BUT it doesn't stop there either - if you decide you just want pure HD500 with amp models and cab models to go direct into the Vetta's power amp, you CAN leave all your cables connected as per the 4CM above and simply not include the FX Loop effect in the HD500 presets you want to use like this. As the Vetta's pre-amp section will be bypassed because the HD500 preset currently in use will not have an FX loop and the signal flow will be effectively as follows

Guitar > HD500 Input

HD500 Left Output > Vetta Left FX Return/HD500 Right Output > Vetta Right FX Return

If using a Variax with this type of setup, you would use the VDI input on the HD500 rather than on the Vetta.

You can have an empty preset (or effectively a bypass preset) in the HD500 which would allow you to use just pure Vetta sound.

Tone Suck should not be a real issue.

SHORT ANSWERS HERE

The above setup isn't just restricted to the Vetta - any mono amp with serial FX loop can be wired like this and another amp's power amp section can provide the other half of the stereo if you wish.

1. In a shorter answer to your first question - the best way in my opinion  to plug the HD500 or any modeller into the Vetta if you're not interested in the possibilities laid out above, is to go direct from the HD500's outputs into the L/R FX Return sockets on the Vetta
2. In answer to your second question - a 'dummy jack plug' is simply a 1/4" jack plug with nothing connected to it.

Regards

Nick