Your project sounds quite interesting, and it is good that you are familiar with induced drag! Judges will be impressed by your understanding of the different types of drag experienced by an aircraft. If you haven't already, make sure you study the other types, as I'm sure you'll run into more than just induced drag fluctuation in your experiment.
The best way to show tip vortices is to send a stream or series of streams of smoke at the aircraft from upwind in the wind tunnel, like you can see here in these examples:http://www.tritrack.net/gallery/main.php?g2_itemId=240http://www.usfamily.net/web/stauffer/debate.htmlhttp://www.oocities.org/ieee_tpc/ieee_images/AeolianVibration1.jpg
The effect you want to illustrate needs a good amount of visible airflow (smoke) so that it is clear, like in this full-scale example:http://www.efluids.com/efluids/gallery/gallery_pages/cropduster.jsp
You can find readily available wind tunnel smoke source equipment online, or you can try to make something yourself. Be sure you seek supervision and/or an expert if you attempt to fabricate your own, though!
Also, make sure that you have a model small enough for this to work properly in your relatively small wind tunnel. You need to make sure that your experiment incorporates what are known as "3-D effects." Basically, this is the concept: in a wind tunnel, you only have an approximation of real-flight conditions, because the model is in a box rather than the atmosphere. The wings do not have an expanse on either side of themselves, due to the wind tunnel walls, and so the effects of airflow from the bottom over the wingtips to the top tends to be reduced or not even present in wind tunnel tests. However, this effect occurs invariably in actual flight conditions, and so "3-D effect corrections" are applied to data retrieved from wind tunnels. I suggest that you look into 3D effects, and other wind tunnel-related aerodynamics in order to properly design your experiment.
As for your second question, I do not believe you can simply measure the effect of the standard airflow, then that of your drag-reduction airflow, and then take the difference. The proper methodology would be to measure the model under the effects of both
airflows, since that would mimic the in-flight conditions. Does that make sense?
You're off to a really good start. I highly recommend the following book for further study to enhance your understanding of aerodynamics: The Illustrated Guide to Aerodynamics
2nd edition, by H.C. "Skip" Smith. It really makes technical concepts, such as those you are studying, accessible to the student.
Good luck with your project! Feel free to return to Ask an Expert with your future questions.