When you select and execute an approach ext on DEP/ARR 6R you will have an altitude constraint over that fix. VNAV will still provide path information so as to arrive over that fix at the constraint altitude. You will be off the LNAV track while on a vector and as the FAF is now the first fix with a constraint then I assume the VNAV path is constructed as an idle thrust descent with applicable deceleration segment to that fix along the extended LNAV track. As such the information is still of use, you will still intercept that path at some point and still need to be at the FAF at the intercept altitude so it is still valid as a guide. Use all available info as always, DME , drag circles etc.