VSA stands for a Vessel Sharing Agreement..
A vessel sharing agreement is usually reached between various container shipping lines who agree to operate a liner service along a specified route using a specified number of vessels..
It is not necessary for each of the partners to have equal number of vessels.. The space that is available for loading and discharging at each of the ports of call is shared between the partners..
The quantum of space that each partner gets may vary from port to port and could depend on the number of vessels which are operated or placed by the different partners within the agreement..
A vessel sharing agreement is slightly different to that of an alliance in that, a vessel sharing agreement is usually dedicated to a certain trade route with terms and conditions specific to that route, whereas an alliance is more global in nature and could include many different trade routes usually under the same terms..