I shall make the following assumptions of your world.
- Graphene and its production are mature in your world. This allows for readily-available super capacitors and large, stable sheets of graphene layered as much as 1 mm.
- Plasma weapons utilize a form of "cold plasma" and rely mostly on the concussive force of high-velocity gas particles, and less on raw heat distribution. These plasmas therefore are in the thousands of K in temperature.
- The soldiers in this case will wear armor equivalent in thickness to a level IV vest, at 20mm thick.
Keeping those things in mind, we shall construct your armor.
Modern bullet-proof vests are being designed in part around non-Newtonian fluids. Oobleck is an example of one such fluid. If you hit it hard and fast, the "fluid" hardens and deflects the pressure of the impact over the entire area of the fluid, thereby stopping your hand and not allowing you to penetrate it.
Taking a modern example, such as the Moratex Institute of Security Technologies liquid armor, into consideration, the technology currently boasts being able to stop an object traveling at 450 m/s when fitted in a vest.
This isn't protection for our cases, bit we want this material for its impact-distribution over a much larger area than most other materials.
Graphene is a material made up of only carbon atoms in a single layer. It has tons of useful properties, among which is great strength and impact resistance. Research groups at Rice University and the University of Massachusetts carried out an experiment using between 10 and 100 layers of graphene and high-velocity objects. They "shot" tiny gold fragments at the layers at up to 3000 m/s. These layers survived these impacts and showed twice the stopping power of kevlar.
100 layers of graphene is only 100 nanometers thick. A million layers of graphene is 1 mm thick. We have 20 mm to work with here.
Ceramics are very good at turning impact energy into breaking energy. When something hits a ceramic and it shatters, a lot of that impact is deflected into the shattering process.
However, ceramics also have awful capacity for heat transfer. This is good for us. When getting hit with relatively high-temperature plasmas, the ceramic layer will keep the the human on the other side of at least somewhat safe.
Lets add up our pieces: high-strength graphene, impact displacing sheer-force fluids, and heat-resistant ceramics.
Theoretically, if you take the fluid and wrap it in a couple millimeters of graphene you have a very strong vest that can take high-velocity impacts into the several-thousands of meters per second before failing. Not only does it absorb those impacts, it deflects them across the volume of the liquid in the vest.
Whatever impact force makes it too deep into the vest proper can be further deflected by ceramic plates placed in pouches in the vest. Using multiple plates ensures multiple potential chances for survival. If the vest is hit with high-temperature plasma, the ceramics give the wearer a chance to make it away with burns as opposed to molten bones. Lastly, the liquid in the vest will have water content, which will provide for a layer of high-heat-capacity material as well.