I did a simple calculation based on Japanese economic miracle “income doubling” plan which made an assumption that if they want to double the income of the country within 10 years time, they will need to make a GDP growth on average 10 percent per year.
Vietnam GDP growth rate is actually much lower than that but assuming that they manage to make that kind of growth, their GDP in 2017 is 239 billion USD, using compound interest calculator, they will reach around 620 billion USD in 10 years.
Thailand 2017 GDP is 455 billion, GDP growth rate is around 4 percent (more or less) per year. If we assuming that this 4 percent growth rate is true (and that we didn't burn ourselves down to the ground yet), the GDP will be 666 billion USD in 10 years.
So 10 years probably not quite enough for them to surpass Thailand (and this is a very optimistic estimation as in reality their growth is about 7 percent.) However, if they maintain that level of growth for even a couple of years more, they will most likely surpass Thailand in raw economic power.