how can i add devices to monitor not on same vlan?

You must ensure that IP traffic between your PRTG installation and your devices is possible. If PRTG resides in a different VLAN than your devices and both VLANs are blocked from each other, it won't work.

Well regarding Vlans you have multiple options.

The most easy way is that you build a (out of band) management lan. Basicly this means that you manage your devices from a separate subnet and not using the production / service front end. This can be build with more subnets which can be routed transparant in a cloud or routed through firewalling. In one or more subnets you can setup your network devices you like to be managed.

If you have multiple devices from different customers (in a datacenter lan) you can trunk the different vlans to your paessler management station. Every subnet or vlan needs to be configured as a logical interface on you paessler management station and maybe you should define static routes towards the different interfaces to reach everything as well. So you have more interfaces which are listening to SNMP and sending SNMP requests.

The last option and that is not specific the thing your asking for is that if you are using out of band management and you access a router or switch you see all the interfaces which can be dedicated assigned to a any specific Vlan. This is also the case for a multihomed unix / windows server. You can see the interface connected to vlan 1 and an other connected to vlan 2 as example.

As last keep in mind that when you trunk all vlans to a paessler management system you have a security leak because normal a vlan separates different subnets and on this particular place you bundle them together. On the other hand as long as you dont route the vlans there is no possibility that the management station will forward a data frame from vlan 1 to vlan 2.