Tiger, not buying, the keyword is "developing". Producing components at home as much as possible will bring down the cost. What we need is putting pieces of a puzzle into a picture. I don't feel ashamed if we copy China way of doing business.
The 5,000 ton hull can be assembled by the new Ba Son shipyard. Since a decade we make both active and passive radars. So we don't start from zero. What's missing is ESA. A part of the $1.2 b funds should be allocated to develop such radar later be able to operate on ships. Viettel and other domestic companies make a whole range of software, including for air defense, friend-foe identification. We produce Kh-35 antiship missile. The range is said to be extended to 300 km. We make ship artillery, including Gatling barrel gun.
there are lots of other things we need to import, from diesel engines to missiles. We can buy components we need from Russia, India and other friendly countries or is it a problem? I am willing to bet, a Vietnam made air defense destroyer would cost less than the sum you mentioned.