Thank you for contacting www.vietnamvisa-easy.com,
Per our experience, there will be no problem for one-day overstay. Please do not worry at all. However, if you stay in the country over than two days, the fine will be applied. For more than 5-day overstay, you might be denied to exit the country and have to go to the immigration department to explain about the case and be listed in the blacklist of the immigration.
We do hope the information is clear and helpful for you.