Have you got the latest firmware
The specs call it an ADSL modem/router, with the fact that WAN2 can be used connected to a VDSL modem the way you have it sort of dropped into the middle of the description. Note the rest of the description is all about ADSL.
The Vigor2830 is our flagship ADSL router/firewall range, winner of the PC Pro wireless router award for three years in a row. This latest series includes support for professional features such as VLAN tagging, Gigabit Ethernet and WiFi (ess models). The Vigor 2830's secondary WAN interface can be connected to other modems now or later (e.g. a second ADSL, a VDSL/BT Infinity or cable modem etc.). Packed with features, the Vigor 2830 offers truly comprehensive ADSL connectivity and security.Link
Compatible with all variants of ADSL (including ADSL2+ and Annex M) the Vigor 2830 can also be used for cable-modem and leased line applications using its additional gigabit ethernet WAN port.
. That suggests to me that the faster speed on WAN2 may be a firmware upgrade from the original.
Note that there are three versions. Single band, dual band and the 2830n v2. If you haven't checked which yours is that you do so before flashing the latest firmware, as there could be relevant differences.
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