Heidi,
If the main focus of your trip is to visit the Jungfrau, staying in Lauterbrunnen will be the easiest solution, as it will add both time and cost to travel from Mürren or other locations as they're on the opposite side of the valley.
Whether the Jungfrau trip is worth the money depends mainly on the weather. If you're there on a clear day, the views are spectacular, however if it's fogged in with gale force winds (which was the case on my last visit), the views much less appealing and you won't want to spend any time outside. However, you'll still be able to see the Ice Palace and other indoor displays as well as buy some genuine Swiss souvenirs in the pricey shops. Note that the highest point is at about 12K feet, so keep that in mind if you have any altitude issues. Even climbing stairs will be more of any effort.
If you're planning to buy any Swiss passes for that area, you might consider the Jungfrau VIP pass, as that provides a "free" trip to the Jungfraujoch (but be sure to check the forecast before going).
Regarding your trip from Venice to the Berner Oberland, one of the quickest trips will be a train departing Venezia S.L. at 08:12, arriving Interlaken Ost at 14:28 (time 6H:16M, 3 changes at Venezia Mestre, Milano Centrale and Spiez). I used an arbitrary date of 15 May to search. If you'd prefer two changes with a slightly longer travel time, there's a train leaving at 07:50 that also arrives at 14:28. If you're buying P-P tickets, you may only be able to buy tickets in Italy as far as Spiez. If that's the case, just go inside the station in Spiez and buy tickets to Lauterbrunnen. You'll have to change trains again at Interlaken Ost and board the small Berner Oberland train (blue & yellow) for the short and scenic 20 minute trip to Lauterbrunnen. Arriving at about 15:00 should still give you enough daylight to do a walkabout and a bit of exploring.
When you arrive in Lauterbrunnen, you'll see a paved trail running diagonally to the left from the station up to the road. If you take that and turn left at the road, you'll find the Hostel on the left hand side (as I recall). It's not far from the station. If you need a bit of refreshment after your long journey, stop at the Horner Pub for a pint. I quite enjoy their "pub grub" also.