Hi there,
Vizslas are prone to separation anxiety, and expect to be treated as a member of the family. They're not suitable for households where they would be left alone for most of the day. (which I kow isn't really the case with you).
Although they should be taught to enjoy their own company while they are puppies, 13 weeks is just a baby, and 6 hours is rather a long time...
If you don't have one, a crate is useful for keeping them out of mischief when you?re not there, I'd also recommend the radio for company. My dog liked a hot water bottle in a towel to snuggle up to too, but I don't recommend this for "chewy" types, not safe.
Can you ask someone to sit the dog, or at least pop in? That way the dog learns some time alone, but has a bit of re-assurance halfway through so they don't panic you're not coming back? Plus you can relax more and enjoy your night without feeling guilty?
When I was helping re-homing dogs, mainly adults, the rule was not to let them go to someone who would leave them alone for more than four hours at a time. Now as a dog owner I know this isn't always possible, you need a life! But certainly during the settling in period, I feel there's a commitment to the dog to do it right. Plus, a clingy, anxious dog, is a pain for you with all the chewing and howling. Probably better to take the time when they're small to build their confidence.
A good training tip to avoid/deal with seperation anxiety is to ingore them when you come in the house from being out, for at least five minutes, before looking down and saying "hello xxxx" and giving them a fuss. Hard when they're dancing around your heels, but it communicates that you are free to come and go from your house, you are not specifically leaving them and coming home for them.
My dog used to do the opposite to Little B's! He would wait till we'd gone and howl, then go quiet on hearing the car come back and then yap to greet us, I had no idea he was howling until a neighbour told me - although the chewed furniture should have been a giveaway... The ignoring tip worked well, as well as him just "growing" up.
My parents have the wussiest pointer, I think gun-dogs can be rather sensitive