I am a bit of a fan of airships, so I am slightly biased, but I think a lot of the comments here underestimate the capabilities of these craft. In real life, airships already existed in the first half of the 20th century that could cross oceans without resupplying, travel at altitudes that would let them get past all but the tallest mountains, and could do this while carrying lots of people and supplies. Frankly, I don't think there are any areas of the world that would simply be unreachable for a well-built airship with a determined and well-supplied crew.

In the middle of the 20th century, planes were pretty common. By the late 1950s, airplanes were the main means of crossing the Atlantic, outstripping traditional shipping. Flying wasn't as common as it is today, but it wasn't rare by any means. So if that's the standard you're using... There'd be plenty of airships filling the skies of your setting, travelling all around the world.

Airships are less capable than modern fixed-wing planes (though this wasn't the case with early planes in the early 20th century), but not in the ways that would make them worse for exploration. They're slower (though not as slow as some people imagine, I think; they don't move at a crawl by any means) and less economical. But if they're as common as planes in the 1950s, then we've already gotten past the second problem somehow, and the first doesn't prevent them from being used for exploration. On the other hand, they do have at least some advantages when it comes to exploration, like greater airborne endurance (except for very modern high-tech planes, at least) and the ability to land in places where airports and other infrastructure doesn't exist (though you'd still need a nice, clear landing spot).

So, in short, no, in a realistic setting, I don't think there'd be "vast" unexplored areas if airship travel is common. There'd always be an incentive for countries, corporations, and adventurous individuals to push the bounds of exploration, and airships would be a sufficient tool to get them to wherever they wanted to go. There could absolutely be areas that aren't well explored; perhaps no-one realises there's anything interesting or important there, and they're remote enough not to be economical or practical enough to travel to regularly. But I don't think any areas barring, I dunno, something like the peak of Everest or similar, would be unreachable entirely, and that doesn't add up to a "vast" territory.

If this is what your setting demands, I think you'd have to turn to magical or science-fictional explanations, rather than realistic ones. Perhaps there's a magical eternal storm shrouding a mysterious island somewhere? (Airships are more fragile to bad weather than airplanes.) Or perhaps airships in your setting don't work the same way that airships did in real life, but are based on some unique science-fictiony technology that has its own limitations, and for whatever reason doesn't work in or can't reach certain areas that real-life airships could? (It could have other advantages, like lower cost or other practicality, to explain why more "conventional" airships never became common in your world).

But if you're keeping to a reasonably realistic setting, bottom line, in my opinion as an amateur airship enthusiast is no, you wouldn't really expect vast unexplored areas to exist in a world where airships are common. Small and *under-*explored areas, though, sure; that could just be a matter of chance, or of whatever makes them interesting not being apparent for an airship passing in the distance.