The strategy of inferring the meaning of unfamiliar L2 words is one of the lexical processing strategies employed by L2 learners. Inference is most likely to occur when L1 and L2 share common linguistic properties. In the so-called Sinosphere, Sino-Xenic vocabulary—Chinese loanwords in each language—often triggers this type of inference. This study investigates how Japanese learners of Vietnamese apply their knowledge of Sino-Japanese (SJ) vocabulary to infer the usage of Sino-Vietnamese (SV) vocabulary. We examined Japanese learners’ understanding of the syntactic properties of twenty SV words that correspond to SJ forms with differing syntactic properties. The results indicate that Japanese learners tend to judge sentences constructed according to Japanese syntax as grammatical, regardless of whether they are considered acceptable by native Vietnamese speakers.