Howdy,
I know you said not to send JUNOS manual links, but they've actually done a reasonable job of documenting this feature as of 9.0. I know earlier versions did not give nearly as much detail, but if you look in the VPN guide index, it gives you the main use of filtering IP traffic, but also gets into the label consolidation a little, as well as offering the use of the vt- interface as an alternative.
http://www.juniper.net/techpubs/software/junos/junos90/swconfig-vpns/filtering-traffic-based-on-the-ip-header.html#id-10978770
The limitations I mentioned (and a few more) are listed here:
http://www.juniper.net/techpubs/software/junos/junos90/swconfig-vpns/other-limitations.html#id-10993840
As far as using it in every routing-instance, I believe it should be safe. As I mentioned before, I do not remember any scaling problems. However, I will say that I know several VPN service providers who add VTL as part of their standard routing-instance configuration.
As for scaling issues on the "Route Processor" I would expect VTL to help scaling because the router does not have to allocate as many labels (assuming you meant the Routing Engine). If your question was more about forwarding performance, this should not really be a problem either because of the design of the Packet Forwarding Engine where all of the forwarding is done in hardware.
I believe one way to look at it could be that VTL implements the LSI (Label switching interface?) in a similar way to aggregated SONET or aggregated Ethernet interfaces to serve as a software place holder for a second lookup in the PFE. An extra PFE lookup in most Juniper hardware is relatively cheap from a performance perspective since they tend to over-engineer for extra lookups for various reasons. However, the type of lookup available is limited by the interface on the PE-P (or PE-PE link) "core-facing" interfaces, so either the router can forward with no problems or it just won't forward the traffic at all, hence the list of interfaces listed with limitations.
The biggest 2 cases where VTL or vt- interfaces must be used are:
-when broadcast media is put in the routing instance, because ARP functionality does not work with MPLS natively.
-to filter the IP (v4 or v6) traffic egressing the PE-CE link.
Other than that, it is generally a VPN network optimization feature.
Once again, I hope this was everything you were looking for.
#vt#vrf#vrf-table-label#routing-instance