For reasons still unknown, Steam has been banned in Vietnam.
One possible explanation for this curious move, as pointed out by Vietnamnet.vn is that local game developers have recently complained that its difficult for them to compete with the thousands of international games on the platform. Games developed locally are subject to government approval and this same process doesn't apply to other international titles.
GamesIndustry.biz has also speculated that the ban is due to "Valve's business operations".
As of the time of writing, Steam hasn't confirmed the exact reason why the platform was blocked for users within the region.
Thanks, Eurogamer.