Taking a Switch on vacation from EU to USA. What region eshop will I be able to access?

It doesn't matter where your Switch came from. They're all the same, and region-free. The only thing that matters, is which region your account belongs to.

If you have a Japanese account, it will only be able to access the Japanese eShop, no matter where you are in the world.

If you have a German account, it will only be able to access the German eShop, no matter where you are in the world.

etc.

In order to access another eShop, you will either need another account from that country, or you'll need to change your account's country setting. Note that you'll have to use up any and all funds remaining on your account to do so:

Your Nintendo eShop account balance does not carry over when you change your Nintendo Account country. You will need to use the remaining funds before you can change your country.

If you intend to change regions often (or temporarily), I recommend using multiple accounts. You can easily download games on one account, and play them on another. Just make sure both accounts are on the same Switch, and that the Switch is registered as the "primary console" for the secondary account.

The only downside to having multiple accounts from different regions, is that the News channel will display the same news from different regions. You'll also have to select an account whenever you want to launch a game or the eShop (as opposed to simply defining a default account).