firstly i would like to tell you that headache giddiness and vomiting
cannot be due to spondylolisthesis
at L5-S1 and as you said it can be managed conservatively unless there is any neurological deficit.
giddiness and nausea on forward bending could be due to haemodynamic changes like orthostatic hypotension
or could be because of cervical spondylosis
you need to check your blood pressure and also your cevical spine (neck) to rule out the cause of this.