League-of-legends – Rift Herald and Baron

The only role that can viably buy an early Vampiric Scepter is a jungler. This is because lifesteal procs off auto attacks, and in early game you should not be auto attacking creeps because it will push your lane too much.

One of the main ideas behind stacking Doran's is that it provides a cheap advantage for early game harrasment. If two champions with similar base stats are harassing each other, the champ with 2 Doran's will have greater sustain in lane and most likely will win the harrasments. Having more health allows you to win the skirmishes against your opponent, survive jungler ganks, last hit easier, and stay in the lane after a failed jungle gank or harassment.

This does put you behind for getting your first big purchase item, however given that a B. F. Sword (the most common big purchase for AD champs) is 1550g, spending an early 475g on a second Doran's, and consequently getting greater CS (note that after ~25 CS the dorans has paid for itself) and champ kills, can actually allow you to get that B. F. Sword sooner.

This is a very broad question . You're not looking for a time or a level, but rather a one condition: You know where enough of the enemy team is that you're sure your team won't get jumped while killing Baron.

Typically, such awareness comes in the form of just having killed a few of the opponents. Then, you know exactly where they are: waiting to respawn.

Outside of that, ward placement and other general strategies for gaining map awareness can give you an idea of where everyone is. If you see three opponents on the bot lane countering a minion advancement, you can be pretty comfortable with the idea of going for a Baron kill even when you're not sure where the other two opponents are.

Keep in mind, however, that on top of knowing where they are, you want to be sure they can't get to you in enough time. As your team gets higher in level, they will need less time to down Baron. You'll want lots of time in earlier levels and possibly as short as 10-15 seconds at higher levels.