As far as I'm aware (at least on the layer 3 switches I've dealt with) you can't get per-VLAN throughput statistics by monitoring SVIs/VLAN interfaces.
If you want per-VLAN stats (assuming you're trunking multiple VLANs over a single interface and want the stats for one VLAN) you may be able to poll the CISCO-CLASS-BASED-QOS-MIB if you're using policies on your physical interfaces.
I haven't found another way to obtain per-VLAN stats - I'd be happy to be corrected.
Of course it may be viable in your situation (depending on the connectivity) to simply monitor the physical ports to which customer routers are attached. (or maybe not, hence your VLAN question)
On some Cisco L3 switches, you cannot measure traffic via SNMP on VLAN interface since it shows unreal speed. There is no workaround. If you cannot measure port connected to customer it will be a problem. If your L3 switch supports NETFLOW there is a way but it may be costly to implement. From NETFLOW database you can do calculations of customers speed.
In any case, if you chose this there are lot more different things that you can analyze with NETFLOW database.
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