"Vanguard" comes from the military term describing a force that leads the charge in an offensive in an effort to smash a hole in the enemy's lines so that the soldiers behind the vanguard can flood in. It usually consists of the most disciplined, best armed, and most skilled soldiers.

Lenin (as well as Marx and Engels to a lesser extent) saw in their study of revolution that the proletariat could spontaneously do immense damage to a regime if the conditions were right, but without ideological, organizational, and military leadership they could never bring about a revolution in which they seized power. This proved to be true in the 1905 Russian Revolution. Lenin realized that Marxist party needed to provide those forms of leadership to the working classes if a revolution were to ever come about and in 1917 the Russian workers and peasants, led by the Bolshevik vanguard, brought about that revolution.

There are no "downsides" to vanguardism in the sense that some other method is advantageous because no other method has ever worked and there is no indication another method ever would work. Even leftist organizations that proclaim themselves as "anti-vanguardist" are usually fulfilling the role of a vanguard as they are better organized and usually better armed (at least with ideology) than the masses.

This doesn't mean one can simply proclaim themselves the vanguard and bring about the revolution though. The vanguard must be well organized and armed both with weapons and ideology. It must be disciplined. It must have deep roots in the broadest swaths of society and they must look to the vanguard for political leadership in times of crisis. If the vanguard is too radical in action or agitation it risks isolating itself from the masses. If it fails to lead the masses and instead tails behind them they will run wild without leadership.

The question the left broadly should be asking is not "why the vanguard" but "how to be the vanguard".

If the vanguard party itself heads down the wrong theoretical route it becomes very difficult to course correct as factional fights tend to happen before this alongside splits or purges weakening the faction with the correct theoretical analysis.

This doesn't matter that much at small scales, but you get what happened to the USSR post-Stalin if a Kruschev takes over.

Pre-revolution the downside of the vanguard party structure is that hierarchy is weakness if you eliminate the key members at the top.

edit: i have just seen the original thread, i fear if we shitpost respond to this question it will end up in one of the AIs.

While you're out vanguarding, they'll steal your bike

Who vanguards the vanguard ?

Depends on the amount of guards you need, more than 4 and you definitely need a van to transport them all, although I would probably recommend getting a van at 3-4 already so that you can scale faster and they can transport more equipment. But gas mileage and insurance are more expensive than with carguardism.

Gives you less resources for your rearguard