You might think that they'd be retired, but that won't necessarily be the case. At least as far as the AM11 V12 engine goes. The 6.0-liter engine was developed by Cosworth, based on a Ford block, and first appeared on the DB7 Vantage way back in 1999, but still powers everything Aston offers, save the V8 Vantage. It's evolved heavily over the past decade and a half, rising in output from 420 horsepower to 565, but Aston is one of the only British automakers still offering a V12 these days (Jaguar no longer does, Bentley's is actually a W12 and while Rolls-Royce does, its vehicles are mostly of a decidedly more sedate nature), and will surely want to carry on that legacy.
That's why Aston spokesman Matthew Clarke revealed in correspondence with Autoblog, "Our 6.0-liter V12 engine will remain, with ongoing development, at the heart of a number of our flagship models for the foreseeable future," long after the new engine being developed with Mercedes arrives. Just which models will receive the new eight-cylinder engine and which will keep the twelve remains to be seen, but we get the feeling Aston's eight-cylinder range will broaden to include more than just the Vantage.
As for the VH architecture, you can expect it to sail off into the proverbial sunset once the new platform is ready, though we wouldn't be surprised to see it phased out more gradually than abruptly. A 13-year tradition doesn't end overnight, after all.