VSwap is a memory management model from OpenVZ. It reflects "swap" memory.
When the Guaranteed Ram limit is reached, memory pages belonging to the VPS are pushed out to so called virtual swap (vswap).
The difference between normal swap and vswap is that with vswap no actual disk I/O usually occurs. Instead, a VPS is artificially slowed down, to emulate the effect of the real swapping.
Swap is mainly designed for absorbing peaks in the memory usage.
In case your VPS is using swap memory, consider a memory upgrade.
More information regarding "VSwap" and "Swap" can be found on the following links: