Unless you absolutely need the room, you don’t need a vest over 3L/5L. Check out the Salomon line of hydration vests. They have soft flasks on the front, and are easy to reach. They’re pricey, but worth it. You can also put a 1.5L bladder in the back if you need the extra water. https://www.amazon.com/dp/B01MSWWT5E/ref=cm_sw_r_cp_awdb_t1_038PBbZ814Y7S
Edit: search for “running hydration vests ”. You’ll end up with a better selection of gear aimed towards runners.