You probably trying to achieve a sound closer to a Marshall than a Mesa seeing as thats what Satriani uses and Vai's Carvin Legacy is more Marshall than Mesa. A broad generalization that I use when gauging an amps tone are the power tubes: EL34's = Marshall, 6L6's = Mesa.
So first off, you could try changing the power tubes to EL34's. If that doesn't sound appealing, an EQ in the effects loop can completely change the character of your amp. Depending on how comfortable you are using EQ's, a boost followed by an EQ in front of the amp, followed by an EQ in the effects loop can make the Roadster sound like anything you want it to!
But lets work with what we have first. Lead tones usually sound the best on Channel 3. Here is the range I like to use:
Back of the amp for Channel 3: Recto Tracking, 100watt, FX wherever you want it
Front of the amp for Channel 3: Gain - 1:00 - 3:00
Treble - 10:00 - 11:00
Mid - 10:00 - 11:00
Bass - 11:00 - 12:00
Presence - Off
Vintage Mode, Bold
You might want to bring in a little bit of presence to sweeten up the tone. Try Spongy mode instead of Bold, 50 watts instead of 100 watts, etc. Also a bright humbucker would help too.
I aim for a more Petrucci - like tone but I'm sure there are more experienced Roadster users who can chime in on how to dial in a Vai tone.