It is currently possible to get both Vic AND Isaac to join your community on the same play through, which may reduce the amount of time you need to invest into a second run through the story to get the Vehicle Upgrade/Weapons and Brock/Keesha achievements.
To achieve this, naturally progress the story until this lover’s mission comes up. At this point in time you should have the mission from Captain Logan that’ll introduce you to the Plague Buster - DO NOT COMPLETE THIS YET!
Once you have this set up, progress the quest as you like and pick Vic or Isaac to join you. Then, on your way back to base, detour to Captain Logan and begin his mission - this will send whoever you have with you back to base automatically AND re-trigger the Vic/Isaac quest so that you can easily do it again and choose the opposite partner for both achievements. This obviously isn’t intended and the two never acknowledge each other’s presence at home.
This also helps with the Full House achievement in the event that one person bugs out on you!