To earn service credit during a calendar year, a volunteer must earn 50 points. To become 100% vested, a participant must earn five (5) years of service credit (this is true in 99% of LOSAPs - some though do have alternative vesting schedules).
Therefore, it is possible an individual could be a volunteer for 5 or more years, but not achieve 100% vesting status because the volunteer did not reach 50 points in five calendar years.
In general, the five years does not have to be consecutive - just during the period of volunteer service. However, there are some plans that will stipulate that if a volunteer fails to earn 50 points for a period of time - like three consecutive years - that the volunteer will forfeit what has been earned and have to start over again. These provisions are not uncommon, but not universal. Check your plan document or summary of plan description to be certain.