They do turn up to buy on ebay from time to time, it just takes some patience. Cubase and Logic are fairly popular and frequently turn up on servers which offer abandonware and unsupported versions.
May I ask what hardware and OS you plan to use? This has a lot to do with how old you can do. Last year I set up an SE running 6.0.8 and this winter an LC475/Quadra 605 running 7.5.5. There are some apps which will run on either, but most prefer to run on their own era system. For an 040 system you should stick to later 68k and FAT programs (mid 90s) for best compatibility. If you want to use an emulator you should do it just for historical reasons, because your MIDI timing will be off - its not practical.
What is really great is that when Opcode got dismantled and Vision scrapped, the developers basically gave the program away for all to use. There was an Opcode User's Group website for years which hosted drivers, manuals, Galaxy, and the latest versions of Vision. Version 4 of Vision may have even still been 68k compatible, Good news is that much of the old Opcode stuff is mirrored across the net. One place is the yahoogroup "opcodeusers". It is still active and offers among other things Vision 2.5 for Windoze and Vision 1.02 for 68k Macs. Vision 1.02 works great on my SE, I recommend it.