League-of-legends – Are stealthed opponents visible when capturing altars on Twisted Treeline

Only works if the target is visible per this.

Enemy champions that are not visible to Vayne (through Fog of War, brush, or stealth) will not activate Night Hunter.

If by constructive, you mean you want to make the playing experience as similar to a real 5v5 match as possible to gain real game time experience then there are several things you can do:

3v3:

• 1 mid, 1 top, and 1 jungle (and nobody bot). In a real higher elo game, it is usually always 1v1 top to compensate for the jungler.
• 1 mid and 2 bot (and nobody top). This will closely resemble a game where neither sides have a jungler.

4v4:

• 1 mid, 2 bot, and 1 jungle (and nobody top). Resembles a real higher elo game.
• 1 top, 2 bot, 1 jungle (and nobody mid). Resembles a real higher elo game.
• 1 top, 1 mid, 1 bot and 1 jungle. This will resemble a real game where a team has a jungler and a roamer.

Of course, don't forget about the bots. You can always use the bots to fill into the missing roster to get the 5v5.