There are two ways to do it, one with your computer at the concert ( and I agree), who wants to lug that around, never mind the risk of damage), and one without, which follows:
1. A cassette recorder WON'T give you good sound quality. Believe it or not, take your VHS machine, and hook up the L and R output of whatever you are recording from ( make sure it's the Line Output !! not the output to any speakers in the room !! ) to the L and R audio input RCA plug on the back of your VHS. Use a good tape, and record in Short Play if you also have Long Play - Short play is like using 7 1/2 ips versus 3 3/4 on an old reel-to-reel recorder. Because of helican scanning, the sound recording quality is near-CD ! Just set the VHS going and with a 3 hour tape....
2. At home, unless you or someone is handy with soldering plugs, you will probably have to get a local hi-fi store to make up a connecting cable, with RCA audio plugs one end, to fit into the VHS; and at the other, have the cable going into the single (stereo) audio plug that fits into the audio input hole on your Mac.
3. You need a brilliant piece of software called SoundStudio which I think is the easiest to use, the best interface, and best features. You would just open this and Click a button to start a new recording, and start your VHS playing back the tape. Watch the audio input meter of the software for a while, particularly loud passages in the music, and adjust the volume slide of the software to make sure, as the volume fluctuates, that it doesn't 'go into the red'. When you're happy -(and it's best to slightly underestimate ie have the volume a bit lower than you think in case at some point there is a music peak), close and don't Save that recording, and start again, this time letting it rip to the end. Stop the VHS and the software and save to hard drive.
The beauty of SoundStudio is that when you play it back, you will see if the visual representation of the sound wave goes above the upper line, indicating distortion. Because you recorded at a lower level, it shouldn't. But now, you can amplify any track by varying amounts, check if it stays within the lines, or Undo and try again. This will maximize the volume level of the final recording on a CD. You can edit out any 'waste' before the bands start, and Cut out any/all the recorded sound/chatter/interrruptions etc between songs. If you want them in different order, you could cut and paste them into a new File in the order you want; or if you just want to omit one or more songs, just highlight each by dragging across from beginning to end, and use Cut command.
This will make a CD that can be copied for any family and friends. But as you can see, if you just take the VHS tape and play it normally through a Surround Sound system, obviously, no video to the TV screen, but you will get the sound, the whole lot, pauses, interruptions, applause etc but in great high fidelity. If you decide to do this, and need further help, let me know.