Is this a continuous process, with "steady-state" feed and distillate rates? What is the function of the column in your process? What is the feed composition (continuous process or the charge to a batch distillation)? Have you obtained pressure readings at the top and bottom of the column and at the exchanger outlets?
If continuous, and your column and exchanger pressure drops are reasonably small so that the column is close to atmospheric throughout (thus with negligible air leakage), then any nitrogen present prior to startup would be swept out rather quickly with the vent stream as you boil up MTBE. DIPPR database lists MTBE NBP as 131 F. If you cool a pure MTBE vapor stream down to 80 F, you will get all liquid and no vapor for the vent condenser to handle. The nitrogen purge will serve to maintain column pressure, presumably. If you do not have a vacuum pump or ejector, there is no way you can go vacuum due to flow and suck in nitrogen from the vent line. If you have high pressure drops, pressure will simply back up from the downstream equipment and become substantially higher upstream of the condensers, not atmospheric.
If indeed you have a significant duty in the vent condenser, meaning you have a substantial amount of MTBE in the vapor downstream of the main condenser, then you must have something else entering the column (with the feed or fed separately). It would be something noncondensable or at least very low boiling. You can approximate how much there is, relative to the MTBE. As 25362 points out, you need to know how much of that something else is present to quantify the MTBE flow from either condenser. At 80 F, the MTBE vp is 5.18 psia (what's your source of vapor pressure for MTBE?), so it is indeed only about 1/3 of that vapor. The "something else" is the other 2/3.
Once you identify what that something else is and how much of it is present, then you can do your condenser duty distribution calculations. If that other stuff has very low flow, then practically all your condensing is done at 131 F, with a sharp dropoff at the end of the curve until you pinch off at 80 F. The vent condenser similarly will squeeze out a bit more MTBE as it, too, pinches off approaching the brine temperature. If there is a huge flow of a low-boiler/noncondensable in the feed, then the situation is reversed, with very little condensing in the first condenser and the main load dumped on the vent condenser. As your noncondensable flow goes way up, you reach a point where you can cool but not condense anything in either condenser. I gather your situation is not that extreme, and that the noncondensable flow is small enough so that you do have some condensing going on in both exchangers. But you must have something else present in the feed(s) to the column, and how much determines everything else.
Perhaps you could get flow and temperatures for the coolant on both exchangers, to back out duties. Then indirectly you could calculate how much you have in lights.