Congratulations to your new Vizsla.
The feeding once a day does not correlate with underweight. My almost 3 years old also gets his kibble in the evenings, and it was him who simply stopped eating breakfast. He is a working dog type, very active during the day, falling sleep after dinner.
I would not drastically change a diet after 3 days, let him settle. Adding some high quality treats (i.e. cooked salmon in a Kong) may be able to boost him, but sudden changes may actually have an adverse impact.
Hope you can find whatever is best for him:-)