Don't worry, buddy, we've all been there. Nothing like tanking for 2/8 Panth and 1/9 Vayne.
Also, a note on Maokai- Philostone AND HoG first thing put the RoA too far back, making him absolutely harmless. Besides that, a Catalyst should hold in terms of health/mana/sustain.
Also, a question- Am I better off maxing the sapling first for lulz and harassment or go for Twisted Advance for the root duration? I usually go for Advance, but...
Sapling, in my opinion. They hurt like hell at level 5 if the enemy gets caught by both the land and the explosion. Then the enemy zones himself out of fear of your tiny minions. But then again, I consistently go AP Maokai instead of a tank. And yeah, I rush the RoA first. He's a bit too squishy otherwise. Just try not to throw too many saplings. Actually, if you're careful about using them, you can actually get away with taking Clarity as a summoner spell and not taking any mana regen items.
I usually go with Twisted Advance to assist my sapling burst damage. Throw one sapling on the enemy, root them in place, then the sapling explodes. It does a good amount of burst damage and makes the enemy fear your saplings even more. If they have like 3-5 bars of health left and you have level 5 saplings, go for the sapling, root combination. It'll probably end up killing them or at the very least forcing them out of the lane, even with around 65-70 AP aside from the RoA.