Hey there,
First of all I can also confirm all that is said above, especially about friendliness, safety / low crime and beauty of the country.
I wanted to see how easy it is to buy a vacation home/rental in Montenegro?
Buying process is very straightforward, and as Docperic says, it is advisable to have a local friend or another expat who's been living here long enough, preferably who is familiar with property issues, to confirm the reality of prices, urban documents, building permits... Land registries are accessible online to anyone, so you can easily check ownership rights / existence of mortgages etc. Until a few years ago, lawyers were handling the sales contracts but now Notaries have been "implemented" for that. However it is a good idea to hire a lawyer to check the land records on the day of the signature to make sure no changes have been made to the title deed on the same day (as this would not have appeared online yet). That's about it !
How has it been assimilating to the culture in Montenegro? Can it get boring or is there lot to do? Are people in Montenegro warm and inviting or cold and unfriendly?
Montenegro is at the border of western and eastern Europe, so for us, coming from "the West" there is no big culture shock like you'd have in Asia. It is interesting here to see the various influences (Italian, Austrian, Turkish...) that shaped the country, its food, habits... One of the biggest challenge I have faced, even though I come from the south of France where people are rather relaxed, is that here they are super-relaxed... Which is a great thing for holidays, etc. but can be frustrating when you do business. I like to say that one of the first words you will learn here is "Sutra", which means "Tomorrow". You'll quickly understand. So anyway, you must adapt to this and be/learn to be patient.
So yes people are very friendly. If you like big cities life, you could get bored, here is more of a country-side life, where you can practice almost any single sports, swim, ski, hiking, go fishing, paragliding, etc etc. So if you love nature, you won't be bored. If you like going to the theater every night, you will (except July-August when there are a lot of festivals).
How expensive is it compared to the US?
Try this website for an idea of the differences: http://www.numbeo.com/cost-of-living/comparison.jsp
Here is for ex. between New York and Tivat :
http://www.numbeo.com/cost-of-living/co … ity2=Tivat
All the best!