Currently, in order to fire a heavy/secondary weapon you have to press the button icon near the aiming touch area (bottom right). This makes it very difficult to aim and fire at the same time. Also, as the icon is quite small, if you keep moving your finger from the aiming area to the touch area it's easy to miss, which, if you tap on an object means that you accidentally target that object and lose targetting on your intended target.

An option to allow swapping of primary and secondary weapons over (so that the button icon on the left could be used to fire the secondary weapons) might be away to solve that.

Another option would be to put both the primary and secondary weapon buttons on the left side (and maybe a 3rd button for tertiary weapons?). I think this would be my preferred option. It would make manual firing of weapons and simultaneously changing speed or strafing difficult but it would seem like less of a handicap overall.

You can customize the HUD using the DroidButtons plugin and do exactly what you're asking.

It'd be wonderful if the devs made a more customizable/flexible control scheme for touchscreens, but that's not the case yet. In the mean time, there's DroidButtons.

Thanks, I've just figured how to add plugins.
BTW the android guide to plugins is out of date (in main forum). The correct path should be:
/sdcard/Android/data/com.guildsoftware.vendetta/files/plugins (you still need to mkdir plugins).