USB interface would give you access to better audio interfaces with higher SNR and accuracy recording modes. An alternative that you may already have is the stock sound card on your motherboard. Most audio interfaces have the ability to be configured as Line-IN or Line-out, then configured in Vegas/SoundForge/Audacity as a recording interface.



You set the recording mode for each input, for example 16bit/48Khz, in the sound card management interface and not inside vegas.





Switching your audio device under preferences to Windows Classic, gives Vegas Mixer access to select the individual interfaces separately as a mixer track. To view the mixer in Vegas cntl+alt+6. Check arm for record on each track you want to record and then hit Record on the timeline window. The "real time" track effect chain all apply to each input when recording, for example noise gate and EQ.

This might be cheaper than an audio interface if the quality is acceptable. Again unless your internal PCIe sound card is Pro/exceptional, a USB or firewire interface with music grade hardware will make the floor the lowest. You want the floor below the tape deck floor, so you are getting what the tape has to offer.

This is also how you can setup multi-channel audio out for surround, by setting the playback channels under preferences>Audio Device. Then setting each track in Vegas to the corresponding output channel. If you have no other output channels configured in Audio Device prefs, you will not see any other output options than master default for track output..