As someone else said it's difficult to have the "correct" answer because it's not perfectly clear what your opponent is doing and there is a big difference depending on what he's flying. For the sake of argument lets assume you're co-e with similar airplanes.
If, by your description, you mean that he entered a more level turn vice the vertical opening there are two potential situations. 1) the guy doesn't understand the vertical move and is a grape ready for picking OR 2) the guy is maintaining his e with only a slight nose high turn waiting for you to dive in for the munchies.
To determine if the guys' a grape look at how quickly his nose is tracking across the ground. By this I mean look to see if he's got a lot of turn rate, if so he's generating a lot of useless angles and he's really just burning his e and won't be able to do any significant zoom. In this case simply convert to his six for an easy snack.
So, lets say you look at your opponent and instead of generating a lot of angles he's staying fast and only has a moderate turn rate. This tells you he probably knows what's what. Another variation of this is he initially does a nose-high oblique turn after the merge and then does a wingover to put himself underneath you. The oblique lets him slow a little and reduce his turn radius and increase his turn rate so he can point to a position underneath you. Completing the wingover means he can now re-accelerate in preparation for his zoom into you.
In either case you've got to quickly decide whether to go up or down. If you go down he's watching for you to commit and bury your nose and come screaming straight down thinking you'll just blow him away or work the B&Z on him. He has two options at this time depending on your relative positions.
If you're in front of his wingline he'll probably pull up directly into you for a vertical merge. In this situation you're buried nose low and he's counting on you being fast. After the merge he'll be decelerating and you'll be yanking on the stick trying to get your nose moving uphill. A quick pivot and rudder reversal on his part will get him turned around but he probably won't follow you with pure pursuit, instead he'll use lead pursuit to cut across your turn circle and pick you off as you come up in front of him.
So, what happens if you're behind his wingline? He'll still count on your being too fast to square the corner to get on his six so he'll go vertical in front of you but before you get in guns range. This will put him safely inside of your turn circle and, once you overshoot he'll pivot and rudder over. In this case he'll actually have a lot of angles on you and be close enough to go directly to pure pursuit as you either dive away or continue your pull into the vertical.
OK, how do you cope with this. First, you could play it safe and immediately do another immellman or, if you have insufficient e then do a spiral climb as Humble describes. By doing that, you're maintaining separation and possibly setting up the rope. The spiral also makes him maneuver on his zoom climb causing him to bleed. The downside to going up (oxymoron?) is that you're not keeping pressure on him, you're counting on him making a mistake and overall this may not work to your advantage as he gets more time to live and the possibility of a successful bug.
If you want to stay aggressive against this type of opponent then going nose-low is the way you want to fight but remember you have two or three tricks. The keys here are 1) don't bury your nose, and 2) control your speed. To do this don't just continue your loop and dive straight down, instead roll into a descending oblique turn towards his control zone. Remember that he's maintaining his e but it's going to cost him angles. Also, your spiral means he has to turn harder to stay under you for his vertical move which will help to bleed him a little. Using your slower speed means you have a smaller radius relative to his and can work towards his six. Also, by coming down on him in an oblique turn trick number 1 is built in, use g in the turn to control your acceleration. Your second trick is to slip the plane. Put in a lot of rudder opposite your turn and use aileron to control your bank angle to maintain the turn in the right direction. Trick number 3 only really works in the F4U but drop your gear or use dive brakes if you've got them. Remember as you do this you want to control, not eliminate your e. He's still fast so you'll need just the right amount of e to engage him in a close fight without letting him zoom well above you.
OK, you've got your descent under control, where do you go? You need to be converting on his high six. Now, this is what makes this fight difficult, it's the three bears and too hot, too cold and just right. It's hard to be more specific but you want to stay a bit above him (i.e., don't overshoot in the vertical) but not too high. Too high gives him separation and he can still generate a 90 degree pass with his e. By being in the correct position you should be able to take your choice when he goes vertical, either lag him so you can get a shot when he's above you or lead him and shoot him on the way up. The choice of which depends on the separation you have when he goes vertical. Lots of separation and you want to lead him, little separation then lag him. The point here is to make sure that you don't lag him enough that it gives him the overshoot he's looking for.
Mace
