Citrix has bought Vapps, a provider of high-definition voice conferencing services geared for businesses and service providers.

The $26.6 million deal revealed in a Securities and Exchange Commission filing potentially gives GoToMeeting the added boost of high-quality voice to upgrade its existing voice offerings.

(View a slideshow on the hottest tech M&A deals of 2008.)

Vapps already advertises that its high-definition audioconferencing can be used in conjunction with GoToMeeting. The acquisition could mean integrating the two services to make it easier for customers to get the best features of each.

GoToMeeting can blend audio and videoconferencing with collaboration tools including desktop sharing and chat. It also supports application sharing, attendance reporting, and meeting recording and playback.

In addition to high-quality audio, Vapps brings support for Skype, which Citrix could also add to its GoToMeeting mix. This capability lets Skype users call in to conferences attended by users of traditional phones.

Vapps had a previous relationship with Skype in which it helped expand the maximum number of participants that could join a Skype conference call.

Vapps also supports triggering audioconferences from Microsoft Outlook. GoToMeeting can already be triggered from Outlook and Lotus Notes.

According to the SEC filing, Vapps could receive an extra $4.4 million in the deal if it hits revenue goals.