The Standard Vaccinations include:

Pregnant mares should be vaccinated for Rhinopneumonitis at 5, 7, and 9 months of gestation.  
-At 9 months they also receive a VEWT and Rabies.  
-At 10 months they receive a Rhino/Flu and West Nile Vaccine.

In areas with a history of strangles outbreaks:
-Vaccination must be given before the 6th month of gestation

In areas with a history of Rotovirus outbreak vaccinations must be given at the the 8th, 9th, and 10th months of gestation

If concerned about protection your foal against Botulism Type B infection (“Shaker Foal Syndrome”), pregnant mares are vaccinated at 8, 9, and 10 months of gestation.
In a controlled environment (closed Herd) a single vaccination given 30 days before foaling should be adequate.

You may consider putting pregnant or lactating mares on a good Vitamin/Mineral Supplement.