That could work, and I believe it´s been done several times (...but do also note that there have also been some reports of having difficulty entering Thailand, discussed lately here in the HUBB, too).

And another possible downside could be the selection of bikes available for purchase in Vietnam, compared to Thailand, for example.

To the OP: ask yourself, do you absolutely need to be able to go to all possible countries in that region, especially if it´s your first trip there?

For example, I did a trip on a Thai-regoed bike in 2005, that went from the eastern seaboard of Thailand to Pailin (Cambodia) then Phnom Penh - Kratie - Stung Treng, then crossed to south Laos, then headed north to Savannakhet - Vientiane - Luang Prabang, then negotiated to put the bikes (we had 4 of them) on a river boat, and sailed 2 days north on the mighty Mekong, then crossing back to Thailand in Huay Xai, and finally enjoyed the fantastic mountain roads of northern Thailand on the way back south.

This was about 4000 kms and 3 weeks (but you could very easily double that time & mileage, just depends on how long your holiday is!)... the scenery, climate, people, language, culture, architecture, food, , etc – they all changed many many times on the way, it is quite fascinating, how different things you can see there in an afterall relatively small area. I´ve done many other great trips to these countries before and after, but I still rank that particular trip into my top 5 motorcycle trips ever (maybe the Mekong trip was the icing on the cake). Note, that we only went to 3 countries, and these are all in the ´easily do-able´ category these days.



A couple of pics from the boat trip... and below the so-called "vertical runway" in Vientiane (google it - it´s an interesting story!):

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