I am not familiar with the Vectra 3D, but adverse affects from flea medications are possible. Fleas are easier to prevent than they are to get rid of. It can take several weeks to break the flea cycle when an infestation is present. When you walk outside, they can hitch a ride on your clothing. If you have not already done so, you need to treat your house, yard, and bathe and treat your Maltese before re-introducing him to your home. You can ask your vet what is safe to use. If the infestation is severe, this process may have to be repeated in 3-4 weeks. Since you live in an area where fleas seem to be a problem, I would keep your Maltese on a flea med year round. Also, I would spray around the outside of the house as needed throughout the year. Good luck!