The Barossa Vintage Festival is the kind of event that captures the feel of an entire region. It celebrates the past, through the Ziegenmarket and Luhr's Cottage, and explores modern culture in Barossa Valley, which is known for its wine and wealth of good food.
The Scarecrow trail is the most immediately obvious part of the festival, as scarecrows dot the area. The festival has made a Scarecrow map for anyone who wants to find them all, and you will encounter many of them as you explore the festival.
Once you've finished with that, check out the Festival Memorabilia Exhibition, to explore the history of the very festival through, well, the memorabilia of past festivals. Equipped with the knowledge of the festival's history, you can finally turn your attention to the present.
Nuriootpa is hosting both of the most exciting wine-related events of the festival. The Saltram Underground Tour allows you to get down into a winery's cellar, not just stop at the cellar door. To really get a unique experience though, Wolf Blass are running a wine blending course. Under the tongue-in-cheek Blend it like Blass, you'll get to learn how wines are blended, and then have a go at it yourself. Both events have places all five festival days.
Going a bit out of Tanunda proper, and heading down Elizabeth Street, you will come across Bethany Lutheran church, which is the site for historical re-enactments. The church itself, being 150 years old, has no need of a costume to look the part. There are daily events, including guided walks and interactive displays. On Thursday, there will be a performance by The Amicus Strings, followed by lunch, which sounds like an absolutely delightful afternoon.
If the history of the area is what excites you most, there are two other events you must check out. The Ziegenmarket, on 16 April, starts at 8am and finishes at 11; it aims to recreate the feel of a market from the 19th century, with stallholders dressing and acting the part while selling local produce. This would be well worth the early morning rise. If mornings or markets are not your thing, the Parade starts at 10am on the Saturday, and promises a wonderfully eclectic mix of displays to suit the region.
If I had to recommend one day, the events on the Thursday look the most unique and interesting. It would be a great place to start, allowing you to visit the Ziegenmarket, peruse the Barossa local produce market, listen to the Amicus Strings, and then make it out to Luhr's cottage, where the historic small building will host a light, candlelit meal. If that is not a day to satisfy you, I don't know what is.