I take no stance on vanguards and the scoreboard, but I do want to point out a problem with this statement. Let's say you're playing silver with a relatively good vanguard. A common scenario that I've experienced is for the first 7 or so waves the vanguard has been careening around and basically winning the entire map single-handedly, with a massive score to reflect that. Around wave 8, or perhaps 9 or 10, the vanguard bites off more than he can chew, the rest of the team can't save him, and they later get overwhelmed and lose the mission. This problem can be particularly acute because the play style of the vanguard can weaken the rest of team's coherence as everyone simply starts hunting around the map for anything to shoot and then when the waves get hard suddenly everyone is separated and things go south in a hurry.
See above example. I concern myself with situations where teamwork is required and simply note that very few random teams are capable of adjusting their playstyle starting wave 7 if they've been playing the first 6 waves differently. If it really is a bronze match with a level 20 vanguard just jumping around the map, then you can do that all 11 waves and it really doesn't matter.