Q: Does VSLAM improve or correct the INS data?
A: VSLAM uses the INS to initialise its mapping, but it does not constrain or correct INS drift. The INS data is not altered or adjusted by VSLAM.
Because VSLAM does not constrain the drift, you should aid the INS with an external reference such as GNSS or USBL (or another suitable aiding source). Without this aiding, INS drift accumulates over time and your collection will eventually become unreliable.
For how VSLAM uses the INS at start-up, see the FAQ What is INS-based VSLAM initialisation. INS setup steps (Sensor Settings) are described in the Voyis VSLAM Powered by EIVA NaviSuite Quick start guide.