They do not need vaccinations.
Neither sex is more affectionate. That depends on the individual guinea pig. In general though they are not super affectionate towards people. They are prey animals and can be nervous especially when you try to pick them up.
Cost greatly varies. I spent about $3 a month on hay. I got 50 pound bales from a farmer for $4-8. If you get hay from a pet store be prepared to spend MUCH more than that. Feed stores generally have hay at a median price.
Bedding was about $12 a month. I used hardwood stove pellets. I put a big pile of hay on one side of the cage for them to sleep in too. Actual bedding like carefresh or aspen will be more expensive.
Pellets were super cheap since they only need 1/8 cup a day. Pigs under 6 months will need unlimited pellets though.
Veggies is what I spent the most on. Its been a while and I don't remember how much they cost. They need about a cup of veggies a day. Lots of leafy greens like red/green leaf lettuce, butter lettuce, cilantro, basil etc. Bell pepper daily for vitamin c. Then throw in other veggies like cucumber, tomatoes etc. Here is a veggie chart
One other important thing I want to mention is cage size. The generally accepted minimum is 8 square feet for a pair of females and 10 for a pair of males but I would go larger. They really do enjoy having a big cage. Go as big as you can. C&C cages are a cheap way to get a nice sized cage.