Paying off a loan early isn't a bad thing. Having a credit card for 6 months and then closing it is probably unneeded; pay it off and then keep it as an emergency card.
The key is debt:available credit ratio. Look at this article for example which explains the different elements; the only one you're affecting here is the second, your debt load. If you're not planning on asking for another loan in the next six months, none of this really matters - assuming you are paying it off for sure, in six months, your debt will be gone and your credit score recovered from any hit it takes (and if you get a $1500 credit card and only put $300 on it, it might actually improve your credit).
But having an open $1500 credit card with a 0 balance will probably improve your credit rating, unless you have a really high amount of available credit. It will improve your debt/credit ratio (ie, total $ you owe divided by total $ you could put on your CCs/revolving credit).
This is all aside from the "is it a good idea to borrow money for a 3 month vacation before starting working", which the answer is "Well, not exactly". That's not from a credit perspective, just from a living within your means perspective. If you have a firm job that will easily pay off the vacation, it's probably not a bad thing, but definitely a certain number of people will take this and end up in 'spending bad habits' that last their life. Be aware of that, and if you're just loaning yourself money from the future, make sure you understand the terms of that loan... and are certain you can pay it off.