I looked up the switch you have, and see that it is a Layer-2 switch, so you don't need to worry about Session 15. It's just a method where you telnet into a device and can open separate sessions while there. It's similar to Unix where you open separate command windows or sessions and switch between them. Session 15 allows you to access the router engine, which is separate from the switch engine. You will give the switch an IP address on the same subnet as your VLAN1. There is no need to give an IP address to the VLAN unless you want to be able to manage the switch from a different VLAN.
The switch you have is based on the Cisco 2950, so I looked at that to figure out what you have. Here is the configuration guide,
2950 Switch
As long as you are only using layer-2, you can use VLAN1 to run your traffic, but if you wanted to use Layer-3 routing, which you don't have, you would have to put your traffic on a different VLAN because most of the routing functions don't support VLAN1. You want your equipment on it's on management VLAN so you can still access them in case your LAN goes down from overloaded traffic or some other major problem.
I tried changing the management VLAN from VLAN1, to a different VLAN, and found it's not worth the effort because VLAN1 is hooked into so many different commands and processes.
Check the guide I linked if you don't have the IBM documentation, and use the Cisco "show" commands and maybe a sniffer to see why your 3Com gear isn't seeing your traffic. Make sure you are using VTP transparent mode on that switch. I could probably figure out the problem in a few minutes if I was in front of the equipment, but it may take a few hours doing it over the phone with Tech Support. I could never do that job over the phone, so I don't envy them in the least, but they have saved my butt a few times when I had configuration issues.
I've had many long hours trying to get different vendors equipment to talk to each other, especially if you don't know the gear, so don't beat yourself up because you can't figure it out right away.
Good Luck!
EDIT: I just re-read your detailed post where you want to have multiple VLANS and Trunk ports on that switch. You are probably going to need spanning tree enabled on that switch to keep from having loops in your traffic between the 3Com and this switch. This is where VLAN1 is going to bite you, because VLAN1 will not participate in many switching functions.
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