How to calculate VSAT adjacent satellite interference?

On the downlink, consider the off-axis receive gain sidelobe envelope of your receive earth station antenna towards several nearby satellites either way along the orbit.

In each case take the downlink eirp from that satellite towards you, within the bandwidth of interest, and work out what noise entry appears at the imput to your LNA.  Add up the total downlink adjacent satellite interference.

On the uplink you need access to the uplink earth station details of networks working to adjacent satellites.  In each case take the earth station transmitter powers into the feed (not eirp), per relevent bandwidth, and multiply by the off axis transmit gains of the other networks earth stations.  Add up all the interferences contributions into your uplink at the satellite.

Best regards, Eric.