Full set ready for ultimate action.
Even though the Smersh Vest with Backpack attached has 4 more slots than the High Capacity Vest, you will most likely find the backpack before you find the vest. This means you have to either use it as a backpack or put the Smersh Backpack inside your own backpack, wasting 1 slot.
The High Capacity Vest also only spawns in the Long Barracks, while the Smersh Backpack spawns in normal military spawns, and the Smersh vest at the Helicopter Crash Sites, which means you have to decide what you are going to look for. The High Capacity Vest has less slots than the Smersh Vest with Backpack attached, but the latter is much more rare.
The vest can be worn with a normal backpack.