There are two kinds of folks who use drill presses - those who clamp their work down well and those who are eventually going to get slapped a piece that gets caught (used to be the second type, learned my lesson the hard way and am slightly more paranoid now).
In my opinion DP vises are pretty useful if you do a lot of small pieces that are hard to clamp some other way. Basically think about how you want to solve two problems:
- Solid clamping for all sizes of work you are going to do. This obviously varies depending on the size/shape/etc...
- Initial and repeated alignment. Being able to clamp parts 2-N the same as part 1 can really help your work go easier for some projects.
Obviously there are a LOT of ways to solve all of these problems and DP vises only solve it for some classes of work (and aren't the only solution there).
I'm going to give my stock advice for new tools: sit down and write out ~20 drilling operations with different sized and shaped pieces you would want to do and figure out how you would secure them to the DP in a safe and repeatable way. If you've solved that problem, you're pretty close to done.
I "ended up" with two drill presses (an old - SN 282 - Jet 17" that I got a decent deal on ~5 years ago and an 18-900L I bought last year) the Jet has a metal working style table, the 18-900 has the larger flat WW table. On the Jet I have ~semi-permanently mounted a HF cross slide vise
http://www.harborfreight.com/6-inch-cross-slide-vise-32997.html (apparently the price has gone up, it was $45 on sale when I bought it) - this has been really handy because you can "dial in" the location on more precise drilling operations - the HF vise does have a some slop compared to higher end stuff I've seen in other folks shops but at $45 it was worth it - at $70... less convinced.
I made good use of the HF cross slide vise when I cut all of the rosettes for the trim in our house - I clamped a stop to one side of it and was able to position every rosette in exactly the same place without messing with it. If I was doing it today I'd use the lathe for that project and it would be even faster/cleaner/easier
but given my tooling at the time it was a big win.
Also if you have a way to clamp large flat things to your DP table (as you must
), you can always mount a vice to a auxiliary table later and just clamp the aux table to your main table as needed. If I had "only one" drill press, that's likely what I'd do..
I probably wouldn't buy a vise until I had a real use for it in either case.