To-date this month 80 employees in Technology, including Networks and Cornerstone, based in Newbury and elsewhere, have been targeted.
Prospect Senior Organiser Caroline Hemmington said: "Vodafone is using the fact that it does not recognise a union to try and by-pass accepted practice and make people redundant via the back door.
"We have heard that the numbers being targeted easily exceeded 20, the figure over which the company is required by law to conduct a 30-day consultation period.
"By issuing compromise agreements it is seeking to bypass its legal responsibilities and the processes which exist to ensure people are treated fairly."
Prospect maintains proper consultation is vital to clarify the reasons for the redundancies, the number of staff affected, how they will be selected and under what terms.
Hemmington said: "We have even heard reports from members these vacant posts will be re-advertised at a lower rate which, if true, means these are cost-cutting measures dressed up to sound like redundancy, especially if there are no clear selection criteria.
"In the absence of a recognised union at the very least a company consultative forum should be invoked. Prospect is seeking further clarification from the company on this matter."