There is no rule that says you can only spend xx number of days in xxx amount of time. Anyone that says this doesn't know what they are talking about.
If you want to do
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between Thailand and Vietnam then you can do either Tourist visa 60+30, then go to another thai embassy in the area and get another tourist visa for a second 60+30, then go to Vietnam for 6 months. You'll probably have trouble repeating that every year though because most of the Thai embassies in the region are now limiting someone to 1 tourist visa in a rolling 365 days. So you would need to check with whatever embassy you plan on using.
You could also do like you indicated and do 60+30, then go to Vietnam for 3 months, then get another tourist visa and back to Thailand for 60+30. You may run into the same issues as above though.
You also have the 30 day visa exempt (no visa) that is 30 days with a 30 day extension. But you'll be doing a lot more border bounces to get extra days if you want to try that.