Actually an LF has a decent chance, though it does depend on a few important things.
1) how good the LF pilot is,
2) what shield the VHF pilot has mounted,
3) what type of guns the LF has mounted,
4) how good the VHF pilot is,
After all, having a class 10 shield wont help your VHF if the LF your fighting has mounted weapons that do more damage to it.
For example:
A class 10 positron shield won't last long against class 8/9 pulse, photon or laser weapons.
A class 10 molecular shield won't last long against an LF with class 8/9 tachyon weapons.
A class 10 gravtion shield will be chewed off surprisingly fast by class 8/9 molecular, particle or plasma weapons.
Of course if you have a set up to deal with the most common shield type used it will suffer against opponents who know what weapons you've got mounted.
As for altering the gunmounts, only on the LFs with six gunmounts, make them all class 8 and it should be fine, after all, theres not that much differnece in damage ratings between class 8 and 9 guns.
LFs don't need class 10 guns, they're not suppose to be able to win easily against VHFs, wear VHFs down or fight them to a standstill, yes.
Also, all NPC escorts for transports are disrupter equipped LFs, not nova cannon equipped VHFs, gunboats or cruisers.
