Glad to hear that the HG612 is doing fine and line is fault free, and for the vigor 130 since they did not succeed in reproducing the fault may be it was the modem code that was making it unusable on your line "that would be changed from modem UI or by downloading other codes from Drytek UK website" but I advise you to not try it anymore on your line as it is not worth it so the best solution is to try to sell it.
For the Zyxel VMG8324-B10A I do not know if this model has the line filter inside (which gives better speeds if there is some noise other modems/routers can not deal with) so you may gain extra speed.
This is Zyxel UK website, and there are some fancy looking VDSL models that seems much recent than old ones some with VOIP ports and some without:
https://www.zyxel.com/uk/en/products_services/service_provider-dsl_cpe.shtml?t=c