Vegas is a multi-track audio application (with video) Sound Forge is a multi-channel editing application. I personally need both.
If you need to record and/or mix: Vegas Pro. If you need to master, edit mono, stereo, multichannel (5:1, 7:1, or more for instance). re-draw a waveform and surgical edits down to the sample level: Sound Forge Pro. Some things can be done well on both, it depends on your specific needs.
There are not that many changes to the Veges audio section since VP 10, so upgrading for audio only would not be economical IMO. OTOH, SF Pro, ahs not changed that much, but now is 64 bits, VST 3 support and includes the iZotope Ozone and RX Elements versions.