No, they didnt. I could see that being an issue. For next time, should a Vanilla Porter go into the herb, spice category or the specialty category?
SHV - unless there's something else about the beer that kicks it into specialty (e.g., fruit, unusual fermentables, like honey).
Also, if you do enter it into competition, state the intensity level of the vanilla in the aroma and flavor, but don't necessarily state the exact type of porter unless the base beer absolutely nails one of the three recognized styles of porter.
For example, "Porter with light vanilla aroma and moderate vanilla flavor" would be a good description for a porter which sort of straddles the line between brown porter and robust, and doesn't have a whole lot of vanilla aroma.
The idea is that you want to tell the judges what to expect, and what NOT to expect, rather than making them guess.
As to your feedback in competition, other folks have mentioned possible causes for diacetyl. I'll add another tip: start your fermentation at the lower side of the preferred temperature range and let it rise a bit as fermentation finishes. Diacetyl gets produced during the initial phases of fermentation and gets scavenged up at the end. Cooler starting temperature = less diacetyl, warmer finishing temperature = more energy to clean up diacetyl before the yeast floccs.
Diacetyl and DMS together could be a sign of infection, but often you'll get other off flavors as well - sour, smoky or plasticy. If you're doing everything right with sanitation, it's probably not an issue.
Otherwise, if you're doing everything right by your yeast beasts, using a relatively clean-fermenting yeast strain, and getting good beer, I wouldn't worry about it too much.
Sometimes certain hops and yeast strains can throw sulfur notes which can be mistaken for DMS, especially at low levels. In addition to a 90+ minute, full, open rolling boil, quick cooling and vigorous fermentation will also drive out DMS.
Ultimately, learn to troubleshoot on your own so that you can fix your own problems without having to rely on judge evaluations in competitions. Since you don't have to guess about ingredients and techniques, your evaluations can be much more accurate.
Alternately, hook up with a local HB club, find people in the club whose opinions you can trust and have them evaluate your beer face-to-face. That way, they can ask questions as they taste. If they're BJCP judges, so much the better, since they'll be able to give you tips on what makes a beer competitive in competition.