for the host

I already used ur service once.
I have been working a ODA project in Viet Tri city Pho Tho province since Novem. 2014
When i applied a arrival bussiness visa in last time, the host was a local vietnam company.
after getting to viet tri city, local immigration police checked my visa and requested a invitation letter,
and then, he said this host have no relation with this ODA project, so this is a violation and mentioned a penalty or return Korea.
But, because i should perform my tasks, i have to solve this headache trouble by under money.
so, i wanne ask, how to do for avoiding this situation, what's a need? when i apply a arrival visa.
reference, i have a bidding award licence issued from PHO THO province for the ODA project.