I guess because L1 is a better ship for leveling than Vanguard because of its superior firepower output than Vanguard (i.e. more damage, more exp).
Vanguard is not bad, but you won't be able to bring out its full power at lvl 90-100. Putting lvl120 sailors on Vanguard, it can beat the L1 on 1v1, for sure, given Vanguard and L1 players have the same crew level and playing skills. Why? Because Vanguad's 15*2 is a little bit farther than L1's 16*3, as well as less shell hang time in the air. So when Vanguard is playing run-gun with L1, there is no chance for L1 to hit Vanguard.
But in GB, there is no much chance for Vanguard to play run-gun because UK 15*2 range still sucks compared to other nation. Any BB5 can sink Vanguard in 2 salvos even if you play run-gun, and a Vanguard usually needs 4-5 salvos to sink a BB5, or 3-4 salvos to sink a BB4.
L1? L1 can sink most BB5 with 2 salvos so, take your pick.