Time & Space rift in FF VII world; Cloud & Vincent investigate it, get sucked in and accidentally touch the All-Spark as they end up in the govt. facility. They're locked away, and then hauled out for questioning. The govt. has to revert to a Japanese translator for Vincent (since he speaks Wutanese) and that works quite well.
Cloud and Vincent speak Gaian, a language only the Cybertronians can remember. Their weapons have not been touched since their detainment and Cloud's blade is where he dropped it as no one can lift it. He explains about Gaia, the rift, as well as the tiny little fact that he and Vincent have been brutally experimented on by a crazy scientist who had access to unstable chemicals. The energy from the All-Spark begins to change their bodies into Cybertronians.
They end up as Autobots, though Vincent is viewed with some suspicion due to his red eyes. Can revert to Gaian forms under pressure or when stressed. Weapons resize to accommodate the forms. Vincent is Courted openly by both Prowl and Jazz. Cloud is Courted by the Twins and Rachet. Sam is surprisingly alright with all of this since he's had his own transformation and is secretly dating his guardian Bumblebee.