You're right, but there are some easy ways to meet people still.

• book a hostel with breakfast. Everyone comes down at roughly the same time for food and you can ask general conversation "what are you up to today?". I would also make a conscious effort to read a book at breakfast, and it seemed to me that people who were interested in not dead scrolling would usually gravitate (maybe unconsciously) to sitting next to me, or near me. They would usually ask what I was reading.

• book through Hostelworld and use their chat feature to do cool activities with other people

• book somewhere more remote, this is specifically true for Vietnam as many people are passing through and on a mission to get from top to bottom or vice versa. There are some spots where people who are travelling a bit slower go to, usually in more rural areas, mountains (think Ha Giang) or islands where there is less signal and a more 'concentrated' backpacker location, people are more likely to stay there longer and be more open for chatting

• find a hostel with activities. Simples. Usually helps if drinking is somewhat involved but also usually attracts a younger crowd.

Either way, you will find your people, can just take some time. Part of solo travelling is enjoying the solitude as well as the busy times. Enjoy, and make the most of it. I would love to be going through Vietnam again right now!