- Campus: I've heard that Vassar has a beautiful campus, but Carleton has a marvellous arboretum which sounds great. Vassar also has a farm (?) but I'm not sure what that is... I love the outdoors and I'd like to do some hiking/trekking/ cycling, so it'll be nice to have a college with a great campus.
Well, take a look here: http://vsa.vassar.edu/hudsonvalley/nature/
They don’t mention it on that website, but my D has enjoyed outings to waterfalls, a maple syrup farm and of course apple orchards as well.
- Neighbourhood: I've heard that Carleton is in a very nice small (rural?) town that you can cycle to which has cool cafes and stuff, while Vassar is in an (apparently) pretty **** town with not much greenery and activities so they just stay on campus or take a 1.5 hour ride to NY. Is this a big issue?
The suburb (Arlington) of Poughkeepsie is small but pleasant, with a handful of shops, cafes & restaurants a 2 minute walk outside the gates. There aren’t many activities (not sure what that includes), but it is easy to get to the movies, etc. if you want to. There is plenty of greenery around Vassar
- Weather: It's freezing in Minnesota. But it's also pretty cold in NY. Is the winter depression thing real? I'm really worried about this because coming from a tropical country where it's pretty much summer all year round, I think that if the cold really makes you depressed, I could be in for a real miserable time.
That nobody can really answer. Minnesota is really cold, but at some point cold is cold and if you hate it there’s nothing anybody can do about it. Imo (based in large part on having lived in a lot of different climates) most people adapt to whatever they are in - and college students are generally pretty good at finding the fun part of whatever weather. But: if you are not generally an adaptable person you might be best off looking at colleges closer to the equator…
- Culture: I've read on websites that Carleton has a more silly/unpretentious/grounded feel (especially because of the mid-western stereotype) while Vassar has a more artsy/hippy/sometimes pretentious feel. And I always feel like there's a slightly higher percentage of students who rave about how great Carleton is (and a slightly higher percentage of students that complain that Vassar is 'pretentious') From the videos I've watched, Carleton students do seem a little more silly/take themselves less seriously, but I know it's dangerous to generalise especially since every school will have good and bad types. But as I said, if anyone could tell me the general feel/vibe they got from visiting/studying at Carleton and Vassar, I'll be really grateful.
I can’t answer this one very well, as I’ve never been to Carleton. I will say that it is not a widely held view that mid-westerners are sillier than other regions 
though solid and unpretentious is a typical view. The Vassar students that I know are not at all pretentious (at least, imo!), but I only know a few dozen. Carleton alums do tend to be very passionate about their experience at their school! Since you brought in regional stereotypes, the understated New Englander might have some relevance for Vassar: the school definitely does not do hype in any of its marketing or student recruitment. If Carleton is “Come! You’ll love us!” then Vassar is “You’re welcome if you’d like to be here”
- Reputation and academic strength: Carleton generally has a higher ranking and a higher average SAT score, higher selectivity, and I'm wondering if that means that the reputation it has is slightly better? I know that both schools will give me a great education but as I can't decide, I guess every little difference counts.
Vassar would have more of a ‘name’ - probably b/c of it’s status from it’s ‘seven sister’ days (it was the sister school for Yale, but declined to merge with them back in the 1960’s and went co-ed instead). Really, I don’t think that the differences in ranking or SAT scores or ‘name’ are meaningful for you (or any individual student). Both schools are academically rigorous and somewhat intense learning environments and offer excellent college experiences.
I think that more important for you might be to consider whether you would be happier on a more or less conservative campus, in a more or less structured program, at a more or less extroverted place?