ferry from Vienna to Pula in early June

Do you mean Venice? Vienna to Pula is overland.

While rome2rio.com indicates that through buses from Venice to Pula are infrequent, it appears that Brioni Pula runs buses from Trieste to Pula, requiring less than 3 hours. The company web site is a bit slow, but I found departures at 2 PM, 4:15 PM and 6 PM. Both buses and trains run frequently between Venice and Trieste (roughly 2 hours for the train), so you can definitely make this trip by land if you need to.

However, if I'm reading the Venezia Lines web site correctly, you can get to Pula by ferry on June 7 or to Rovinj or Porec on June 5. Both Rovinj and Porec are very attractive coastal towns. Rovinj is closer to Pula and has fairly frequent bus service there.

I am looking at these ferries in June too, from Venice to Rovinj. Are advanced reservations necessary? recommended?

I took the Venezia Lines catamaran from Rovinj to Venice a few years ago. Catamarans start running in April and schedules vary by day of week. There are no catamarans from Venice to Istria on June 6, but you can depart late afternoon either June 5 or June 7. The June 5 catamaran only goes as far south as Rovinj. The walk from the pier to the bus station is about 10 minutes, with the last bus departing at 21:55 (40 minute trip to Pula).

Also, I booked in advance as a precaution against my travel date selling out. I received a voucher by e-mail that I exchanged for my ticket at the ticket office. The catamaran has airline style seats in the main area and a separate Comfort Lounge for EUR 15 extra. I found the extra cost worthwhile, and it comes with a more generous cancellation policy. For more information, look at General Information - VIP Class on the Venezia Lines website.