Just some quick things @WhichOneIsWill / @Terrachova
You can swap the S5 Gimbal for a S5 gun not a S6 gun. It's a Size 5 item port with a Size 5 Gimbal plugged in reducing the gun port size to 4.
Also, The missiles are still on the top of the Vanguard, the bays on the bottom are for torpedoes. Holoviewer render showcases 2 rotating launchers in each Nacelle which hold 2 torps each for a combined payload of 8 Torps.