Sounds like you should use a many2many relation for this: each of your vehicles can be linked to X services, and each of your services can be linked to Y vehicles. Try using the following fields:

In your vehicle class:

'service_ids':
   fields.many2many(
    'mymodule.service',
    'service_rel',
    'vehicle_id',
    'service_id',
    'Services'),

And in your service class the same thing but in reverse:

'vehicle_ids':
   fields.many2many(
    'mymodule.vehicle',
    'service_rel', # same relation still
    'service_id', # ...but fields of the relation now go in different order
    'vehicle_id',
    'Vehicles'),

The documentation has some notes on m2m usage here: https://doc.openerp.com/trunk/server/03_module_dev_02/