US Taxes

If I visit the US on my vacation and stay there more than 30 days, do I have to pay taxes on my entire salary earned or on one month's salary?

Your question is not clear.

If you are a US citizen or green card holder, you are liable for tax regardless of whether you live in the US or not.

If you are not and you are only visiting the US for vacation or business meetings, you don't have to pay any income tax or file any return.  Your purchases and so on will be subject to sales & state taxes just like anyone else.

I am a US citizen but I make far less than the exclusion amount. With that being said do I pay taxes if I'm in the US for more than 30 days in a calendar year?

As far as I know, there is a physical presence test for exclusion amounts which is that you have to be physically present in a foreign country for 330 days out of 12 months.  And you can choose that particular 12 month period so I would assume that you can still claim the exclusion (but check with a tax attorney to be sure).  See the rules below:

https://www.irs.gov/individuals/interna … sence-test