Yes, you do need a visa to enter Vietnam. Here is more info about that: https://vn.usembassy.gov/vietnamese-visas-and-entry-exit/ The Vietnamese E-visa instructions and application are available online and can be found here: https://evisa.xuatnhapcanh.gov.vn/trang-chu-ttdt INSTRUCTION STEP BY STEP: https://uploads.mwp.mprod.getusinfo.com/uploads/sites/124/2022/07/E-Visa-Application-Steps.pdf
If you have access to fantastic credit/debit cards with no foreign transaction or ATM fees, you can use your cards to pay for services, and food. However, you still need to have some cash on you, because there are a lot of local businesses doing cash only. If you prefer use cash only in Vietnam, there is a $10k cash limit that you are allowed to bring on you on the flights.
You can go to any bank in the city to exchange.
Here is a list of popular tourist SIM card services: https://www.lumahealth.com/travel-insurance/vietnam/best-tourist-sim-card/#:~:text=The%20first%20SIM%20card%20on,free%204G%2F5G%20Viettel%20usage. If you have an e-SIM card, you order an e-sim card for Vietnam on the internet, you receive a QR code, scan it, follow the simple steps and within less than 2 minutes you have a Vietnam e-sim card installed on your phone. INSTRUCTION FOR E-SIM: https://www.traveltomtom.net/destinations/asia/vietnam/best-esim-cards-for-vietnam
There are two ways to get to Nha Trang from Ho Chi Minh. First way, you can always book a flight ticket from Ho Chi Minh City to Nha trang. Second method is travel by bus. This is not a regular type of bus, those buses are high quality transportations which are installed comfortable sleeping and luxury looking cabins with affordable price. Those buses usually take around 6-7 hours to travel between these two cities, and most of the tickets are at night time. So that you go to bed on a bus and wake up in Nha Trang the next morning. You can look for more info and book thru the link below. https://www.bookaway.com/suppliers/xe-nha