Panlogic replaces incumbent Underwired after winning a three-way pitch.
The agency will work with Laura Solomon, VSO head of communications, to develop an email communications strategy for rollout on 7 May.
Monthly newsletters will be sent to people who have signed up to receive information on the site or at events. Panlogic will design banners to support this.
The agency will also develop an email-fundraising strategy to back VSO's face-to-face direct-debit collecting activity, led by Joel Voysey, head of individual partnership development.
Targeted emails will thank new donors and encourage their ongoing support using, for example, live streamed video from a volunteer on VSO's medical programme in Uganda.
This will be followed by a DVD in the post showing global schemes. A further email will offer donors the chance to win a place on a 'Challenge' event.
"We want to convince people when they sign up that it's a good thing to do," said Voysey.