Passport expiring

Hi, we would like to visit Vietnam at the end of this year 2018/beginning of next. My kids passports are expiring in May 2019 so at the moment we are waiting for their new passports to arrive - they should get them back by Christmas. Will it be too late to apply for evisa around that time to travel to Vietnam on 28/29 December? Is it better to apply now with old passports and then wait and change the passport number for visa later? Thank you!