The current Long range radar used in ATC in the US has a max range of 250 NM. I am not sure if they are using the same type in Vietnam and Malaysia.
The Vietnamese Navy said in a statement that the plane went down in the waters between Vietnam and Malaysia, roughly 153 nautical miles off Tho Chu Island in Kien Giang Province, the Vietnamese Tuoi Tre News reported. Malaysia's transport minister said later, however, that no crash scene had been identified.
I don't know if they were in range or if ATC was using fusion.