Now, first let me tell you that vidui is one of the greatest things one can do for oneself. As soon as you realise you've erred in some way, forgotten to make a berachah, missed zman tefillah, forgotten you ate meat a half hour ago and then taken a bite of a snickers bar or anything else at, you stand up before God, acknowledge what you did and ask for forgiveness. This is one of the greatest secrets to a guilt-free life. Once you start down a path like this, vidui turns from something that seems utterly alien, to one of the greatest ways available to put the past behind you and move on in positive directions.
Of course there are technicalities, like if you did something wrong to someone else, you have to first secure their forgiveness before God will put it behind you--this is a much harder challenge than simply confessing your wrong-doings to HaShem. Let's keep it simple for now.
But, imagine you've discovered this miraculous mitzwah, and you already build your life around it, being aware of your actions throughout the day in case of any slip-ups to confess and remove them from your permanent record as quickly as possible, and then you get to the month of Nisan. The whole month, no vidui. Now, on the one hand, great, it's a happy time, we're happy, God's happy, we're a big happy family. Then you go and mess something up, completely by accident, and you don't want to carry that burden around with you, you want to offer it up to God and move on, BUT, it's Nisan, there's no vidui. What do you do? Of course God won't hold it against you, you want to do vidui, but you can't, it's not your fault. But it feels wrong. somehow.
Now take this next piece of personal experience and then try and re-assess. Generally we don't bring up vidui and tachanun at times of great joy, a Brit Milah, a wedding, Holidays, etc. But, last year, on the eighth day of Nisan, the day of my wedding, I was praying under the Kotel (in the synagogue in the tunnels) with the Mekubalim of Bet El, a chaburah that has its roots in the times of the Rav Shalom Sharabi, the RaSHaSH, and all of a sudden in the middle of Nisan, with a Hatan (a groom) in the room, they start belting out Tachanun complete with vidui and everything. (incl. silver shofaroth)
Anyways, this year I've been taking the lack of vidui really hard. So I spoke to Rav Yisrael Avichai, the head of Bet El, successor to Rav Getz, and asked about last year and what happened. He explained to me that the Beit Yosef (Maran R' Yosef Karo, who wrote the Shulchan Aruch and was himself a student (somewhat) of the Arizal) brings down that there's no talmudic basis for this custom of not saying Tachanun in Nisan or for special happy occassions. Further, the Arizal said that people misunderstood the importance of Tachanun and it is so vastly important that except for the days where it is clearly not permitted, people should say it year round. Certainly, Rav Yisrael said, there's no reason not to say vidui if you feel so inclined.
Why'd I have to wait until the second-to-last day of Nisan to ask him ?!? Well, at least I know for next year :)