There is not one thing that you mentioned, that can't be monitored via some tool provided by the Vendor. So, if the vendor can do it, then you can do it also. If there isn't a plugin already made, you can usually just modify or create your own plugin. For example: I had some oracle tablespaces that I wanted to monitor the free space inside the tablespace. The problem was, that the standard check_oracle plugin was reporting the actual free space, and didn't take into account the records that have been marked for deletion. The vedors tool reported the correct free space, so I simply incorporated the vendor's code into the check_oracle code, and bingo, it works.
I'd say most of your checks can be done with check_snmp. i.e. vlan status, interface status, iftraffic (and then graph it with nagiostat too) active connections, switches, power, raid, cpu, .... all with check_snmp.