Add a virtual switch to Vera, make it available in Alexa.
Create a Group in Alexa for "Living Room Lights". Be sure to include the Virtual Switch.
Create a scene in Vera that triggers when the Virtual Switch is turned on. Use LUA to query the dimmable lights. If the lights are at anything other than 100%, use LUA to turn off the "other lights" and then turn off the Virtual Switch when it's done.
Turning on the group would put everything at 100%, the virtual switch comes on, the scene triggers, everything is at 100% so there's nothing to do, the scene turns off the virtual switch.
Dimming the group moves the dimmable lights to a percentage other than 100%, the virtual switch turns on, the scene triggers, the dimmable lights are "turned down" so the fixed lights get turned off, virtual switch gets turned off.
Turning off the group turns everything off.