Difficult and vintage are relative terms, and I think that relativity can vary depending upon the mechanic and LBS in question. Old and vintage may at times be interchangeable, but vintage generally translates as not only old but also good or special or rare or loved.
There are shops that regard bikes from the 90s as vintage, and still others that regard anything steel as vintage. My steel (not cromoly) '86 Schwinn World is probably vintage to some, but there's not really anything special or rare about it.
27" wheels and freewheels are common on vintage bikes, so the availability of replacement parts can be challenging. There is actually a wealth of wheel and tire sizes that are no longer used on production bikes, but the tires and tubes at least are still available. I'd love to meet the marketing genius who decided there needed to be both a 26 x 1.75 and a 26 x 1-3/4 wheel that were different sizes.
If one is keeping it vintage, then it's simply a matter of finding a replacement part and replacing it. Over time crud builds up on vintage bikes, which can make the maintenance work more challenging.
If one is upgrading a vintage bike, then the calculus gets more involved. Took my old Schwinn to my LBS last year, told them I wanted to upgrade, so look it over and give me some options. For safety, there were things I needed. For maintenance, there were still other things I needed. From there, I assessed what I wanted done and what I'd wait on. I went for a cartridge bottom bracket, triple crank, front and rear derailer, new brake levers, and new cables (both brake and shifter).
One of the things recommended to me for safety and comfort was a wider handlebar. This shop deals in some used parts, and this was only $15. It was well worth it, but the stem also had to be replaced when the handlebar was replaced.
When I put fenders on the bike, I discovered that 5mm eyelets are the norm, but my Schwinn has 6mm eyelets. Some light drilling of the plastic fender clips, and 6mm bolts fit just fine.
After blowing out two vintage rear 27" wheels--old bike wheels are no match for old bikes and old bicyclists--I bought a new wheel. Mine was bolt-on, but I wanted a quick release. The LBS had to cut the new wheel down because of the vintage (read narrower) hub spacing.
Next thing I eventually plan to do is move to 700Cs, a cassette (maybe), and Dia-Compe Centerpull Brakes (I like their vintage look). Frame will need to be coldset if I go for a cassette, but this LBS can handle it. That I think is the real key. One LBS may not want to touch a vintage bike whereas another LBS has vintage bikes in their business model. The former would regard a vintage bike as difficult whereas the latter would see it as an opportunity.