Lao and Thai is actually very similar. To be exact, Vientiane Lao and Bangkok Thai is very similar, but as you know if you're from the country from either of these country the dialects and the accents makes it hard to understand. Majority of the formal words are the same in Lao and Thai, but the normal everyday words are a little different, but some you could just tell. If you grew up watching Thai Lakorns then you will understand Thai really well, but if not, then some words will be confusing to you. I can speak Thai fluently, but it feels weird speaking it with the Thai Accent. I can speak Lao fluently also, but I mostly speak English. But you know it just all depends upon the person, like whether you can catch on fast, etc. I went to Laos and in the country part where my grandpa lives, they speak really differently from the normal Lao, but of course I can understand it. But that is just me though because the people that went along with me couldn't really understand it. So it just all depends.