This figure is highest among the six Asian countries polled by the Japan External Trade Organization (JETRO).
Higher revenue was cited as the reason for expansion plans by 65 percent of Japanese businesses.
Another 43 percent saw greater potential and high growth as encouraging factors.
The majority of respondents also said that Vietnam has advantages in market scale and high growth, political-social stability and low labor costs.
65.3 percent of Japanese companies in Vietnam reported profit last year, up 0.2 percentage points from 2017.
The ratio of businesses reporting loss went down 2.8 percentage points to 12.7 percent last year.
Japanese businesses in Vietnam seem to be most confident in their prospects this year among the Asia countries JETRO surveyed.
58.7 percent of businesses in Vietnam expect their profit to increase this year, while this figure is only 47 percent in Thailand, 44.7 percent in Malaysia and 39.5 percent in China.
However, the ratio of businesses reporting profit last year is lower than other countries in Asia, including Indonesia at 65.5 percent, Thailand 67.2 percent, and Malaysia 68.9 percent.
Japanese businesses also said that the top risks in the country were increasing labor costs, an incomplete legal system, lack of transparency in law enforcement and complicated tax and administrative procedures.
They said that Vietnam has a high rate of employee turnover, with 36.2 percent of respondents mentioning this as a problem, higher than in Thailand at 33.2 percent and India at 32.5 percent.
Labor costs account for 20 percent of total costs in Vietnam, higher than the Philippines at 16.8 percent and Indonesia at 16.5 percent.
The localization rate in Vietnam remains low, the Japanese respondents said. Only 14.4 percent of Japanese businesses said they bought material and parts from local businesses last year, lower than in Indonesia at 19.5 percent, Malaysia 20.4 percent, and China 41.6 percent.
This is the 32nd year that JETRO has surveyed business conditions of Japanese firms in Asia and Oceania.
The survey polled 787 Japanese businesses operating in Vietnam in October and November last year.
Japan was the largest foreign direct investor in Vietnam last year, with a total registered investment of $8.59 billion, accounting for 24.2 percent of the total, according to the Ministry of Planning and Investment.