I know about network monitoringsoftware but is there also software that can configure lets say VLAN's on switches of different vendor and types? Is this possible with SNMP?
In theory some configuration is possible via SNMP but it depends on the switch manufacturer. Personally I would lose sleep if SNMP was anything other than read-only on my switches...but that may just be me.
Most shops tend to pick a network vendor and stick with them for compatibility. in that case you use the management tool from that company for configuration. Cisco is much better about having a web-based configuration tool if the command line makes your eyes bleed.
Microsoft was trying to add switches and other network hardware into it's overall System Center management platform. Many companies have released management packs for hardware (all managed through SNMP) but it all looks like monitoring still and not configuration.
Typically you configure your switches by using the management interface provided by the vendor. It helps if you only have one switch vendor.
You CAN configure switches by SNMP -- there are writable portions of the appropriate MIB, so it works in theory -- however I've never seen this set up in practice and wouldn't trust it. The S in SNMP stands for simple, not secure :-)