I'm not getting something here. The VL100 is the Legacy head. In it's basic form, it's a 2 channel amp with a master volume on the lead channel.
The clean channel has one volume control. Set it to where it's not too loud.
The lead channel has a drive control to increase or decrease the amount of preamp distortion and a volume control to set the desired level for that channel. This is the master volume control for this channel. Simply set the drive channel volume at a comparable level to the clean channel.
A THD Hotplate would offer the ideal setup, but it's not absolutely required to achieve your desired result. Just a little tweaking is all you need.
You can simulate a 4 channel Legacy by adding an
overdrive pedal in front of your amp. A Boss SD-1 or Ibanez TS-9 Tube Screamer is the ideal choice. Adding an EQ pedal in the effects loop set for a 5-8 db increase across the full spectrum will offer an additional volume boost to both channels. Here's a classic Legacy setup guide that will give you 4 completely different sounds - clean - warmer, slightly distorted - hard crunchy distortion and a fat, sustaining distorted lead.
Set the clean volume to your desired clean rhythm level. Switch to the lead channel. Set your lead channel drive control anywhere from 7 to 10. This will be determined by your desired distortion rhythm sound. Adjust the lead volume until it's balanced with the clean level. Switch back to the clean channel. Now we'll look at adding just a bit of overdrive with the OD pedal. Set the OD pedal to a "unity volume" setting, with just a bit of overdrive. To achieve this set the gain at about 10 o'clock and the volume at about 2 o'clock. Adjust the pedal levels back and forth until you reach a lightly
distorted tone, while still keeping the volume level the same as the clean channel all by itself. When you switch to the lead channel with the OD pedal on, you'll now have a super hard rock metal lead tone. By adjusting the tone control of the OD pedal, this setup can be quite versatile, essentially simulating a 4 channel amp.
By adding the EQ pedal in the effects loop, you'll boost your volume level for soloing, while your overall tone remains the same as with the EQ switched off.