Can I develop a tolerance to clindamycin that will make it less effective to treat future vaginal infections?

It might. Any bacteria can become resistant to an antibiotic if it is used repeatedly. If you are pregnant and the Clindamycin is being used to treat a vaginal infection, either Metronidazole or oral Clindamycin is likely to work better.
Unlikely. Clindamycin is not commonly used to treat vaginitis. It might be used for bacterial vaginosis but metronidazole(flagyl) is better. Tolerance would be unlikely to develop with Clindamycin use.