I suggest either go with STD, try the VT1 variant, or contact draytek support and get a test of the vector 5 modem code.

One thing to note: just because the BT one is recommended does not mean that you have to use that one. It does say somewhere in the release notes to use BT version unless you have a line which needs one of the others.

Update: looks like the vector 5 (Vigor2860_v3.8.4.3-05NBN_en) firmware is now available as standard on the US site, draytek.com.