What tools or softwares to monitors cisco network (vlan, LAN, WAN, VPN...) ?

You mean like these http://www.cisco.com/en/US/products/sw/netmgtsw/index.html#products

yes, something like that, but it costs money... Thanks

I'd like something for trial or free to test out first before decide to purchase a license. Thanks

http://netsecurity.about.com/cs/hackertools/a/aafreenetmon.htm
http://www.freedownloadscenter.com/Search/network_monitor.html

Just be mindful of what you use, and wait for other input as well.

Several good open source tools you can use are:

Zenoss- www.zenoss.com
Nagios - www.nagios.net
cacti - www.cactu,net

All three of these do a good job of monitoring servers and networking gear. All of them use SNMP to acomplish this. For networking gear i would lean more twords a combination of nagios and cacti over zenoss but zenoss has a simple to use GUI interface for configuration were nagios does not.

I have also hear of Nedi but have never tried it. It looks like it might help you out as well
http://www.nedi.ch/doku.php?id=about