Ken,
The ballpark paces for VO2max efforts in swimming can be estimated using the methods of Sweetenham (in the book referenced by Kevin), and also by the formulas that Urbanchek uses (caclulatable by my widget here -
http://sites.google.com/...og/colorzonetraining ). For folks that are farther from Sweetenhams sample group (world class swimmers), his system might prove a little limiting since it relies on adding a fixed amount of time to your 200 pace to calculate your zone paces -- that said, you are a lot closer to his sample group than most, so it might work out OK. In the investigations that I have been doing lately with the Urbanchek formulas, I am pretty comfortable with what they suggest.
As for the use of VO2max efforts in swimming, I had a very successful colleague that uses liberal amounts of VO2 swims early in the season with great success with distance swimmers. That said, as another poster noted, even the longest event for a distance pool swimmer is probably 1/3 of the duration of your triathlons, so, YMMV.
In my experience, the types of benefits that I associate with a successful introduction and balancing of VO2max intervals (people who race like they've been "pithed" to quote another old colleague) can occur with sets as short as:
3-4 x 150 all out on 1:1 work to rest ratio
and as long as (a set rumored to come from David Salo, one of the worlds greatest middle distance coaches):
3 x 300 as fast as you can hold
3 x 100 same pace
4 x 75 same pace
6 x 50 same pace
1 x 300 all out
all of the above done on a 1:1 work to rest ratio.
The "Salo set" is total money. I can not count the number of times I have seen people post some seriously bad-ass times during the first 3x300s, dial themselves in during the broken 300s (which are a good deal easier since you are holding the same pace as you did on the straight ones), then come back at the end and lay down a total smackdown last 300 straight. I trust you would probably perform thusly.
Bringing you
Tweets @ http://twitter.com/findfreestyle and
Not just a bunch of drills - A Process.