What letters?
You saying letters were sent out? If so how many? to who?
Those letters were to protect the acquiring company, to enable them to pursue the infringers! There's a law protecting an acquiring company that notification is needed prior to purchasing a company. You can google this.
You saying that if VPLM doesn't send letters to supposedly infringing companies, the purchaser can not sue them?
Wait, wait, wait - Did Digifonica send out letters before VPLM acquired them?