I know many Vietnamese Americans who swear it is wise to put money in your passport. The same With Cambodians and Lao. To this day they still do it, and have advised me (white guy) to do likewise for my own good. They will roll out anecdotal stories of the perils of not doing it and what happened to one guy they know whenever I maintain it is unnecessary. I have never tipped an immigration agent in my life except one time in Indonesia and that tip actually avoided an legitimate overstay fine and associated hassle and have been overcharged a few time in Cambodia (but not without intense negotiation of the amount of the overcharge). I am quite certain it is unnecessary in Viet Nam if you are not Vietnamese and I suspect it is largely unnecessary if you are Vietnamese but cannot discount there could be a grain of truth in there somewhere.