We currently have in work a Vigor 2800v modem. We want the ability to offer wireless access to our LAN and internet and we also want to start looking into using SSL VPN for web based file share. I think the 2930vn is the one we will go for.

The 2930vn doesn't have an ADSL modem built in but we do have the 2800v. Can I use the 2800v as the modem and just plug the LAN1 cable into the WAN1 port of the 2930vn?

The 2800 modem is not going to last you much longer, due to forced exchange upgrades to ADSL2 etc. which are not compatible with the 2800. Adding a Vigor 120 will however be a good choice.