The Learjet (v2.8), as good as the model is, is an absolute VAS hog, so here are some tips for improving VAS conservation.
1. Use smaller textures. I've included some in my mod package. This is the best way to save some memory since MSFS creates a clone of the material (including textures) for every animation in a model. And the Learjet has a lot of animations.
2. Disable "High Resolution Virtual Cockpit" in FSX' options. This oddly enough yields about 200 MB of additional free VAS.
3. Lower autogen from "Extremely Dense" to "Very Dense". The extreme setting tends to bleed available VAS dry (not just with the Learjet, mind you), even despite running less varied autogen with lower res textures.
4. Disable flood lights. Yeah, you read that right. Despite Flysimware's optimization efforts, the flood light gauges still contain multiple dimmer stages which increase VAS use. Since these don't make all that much difference at night (console and instrument lights are much more important), it's best to simply disable them by commenting them out with "//" in the panel.cfg. Look for the lines containing "FLYSIMWARE_DIMMER!RADIOS_FLOOD" and "FLYSIMWARE_DIMMER!PANEL_1_FLOOD". While you're at it, also disable the completely unnecessary map light ("FLYSIMWARE_DIMMER!INTERIOR_MAP").
Nota bene: The next release of the mods will contain revised gauges for the lighting which will eliminate all traces of the dimmers.
Other than that, the usual VAS saving tips apply, i.e. stay away from everything with high resolution textures (scenery, clouds, AI aircraft) or detailed mesh, save often and early, use FSUIPC to monitor available VAS, use DirectX 10 mode in FSX, etc, etc...
With all VAS tweaks (except "high res VC" since it affects aircraft using 2D gauges), I took the Learjet on a tour de force along the East Coast in the afternoon all partied up (all lights on) with real weather (quite foul at the moment). Starting north of Boston at 11000 ft, I went south to the tip of Long Island and headed straight to New York City (which is Flight Simulator hell due to AI aircraft and custom scenery objects). Zooming across the city and EWR at 2000 ft, I climbed to FL220 and turned toward Philadelphia and onward to Washington, where I circled at low level (~3000 ft) before landing at IAD. I never dipped below 600 MB of available VAS during the two hours it took to fly the route. I won't dare to say that there are no VAS issues anymore, but it was a definitely positive experience with the aircraft.