The sail's feature you refer to is the "roach". Many sailmakers will recommend a larger roach to increase mainsail performance. Of course, they should also advise you that this larger roach, especially when combined with full length battens, can interfere with the old existing topping lift... That's exactly what happened to me when I first upgraded my mainsail years ago.
Installing a rigid vang is such a great upgrade, you'll thank your sailmaker for selling the large roach mainsail. Not just because of the improvement in boat performance, but because it motivated you to go with the rigid vang improvement.
The best deal on rigid vangs is with Garhaurer Marine. You'll love the price.... just a great value for such a significant upgrade.
Once installed, there's no need to use the topping lift for anything. I wrapped mine around the backstay thinking there might be another use for it some day. Never has. I use the main halyard to stow the boom once the sail is down. If there's a pigtail clamped to your backstay for securing the boom, get rid of it also. Use the halyard.
Okay.... are we clear? Call the folks at Garhaurer right away and they'll get you setup. Full installation instructions including how to create a pattern for the custom made boom and mast brackets that are included with the vang. You'll thank me for this, I promise. Good luck Have fun.