The apex page Book Vehicle(VF tab) is selected actually, But in UI it is seen as Vehicle tab(Custom object tab) is selected.

Please find my extension class (Vehicle search)

public class VehicleSearch{

public VehicleSearch(ApexPages.StandardController controller)
{
 stockid=System.currentPageReference().getParameters().get('id');
}
/* Variable declarations */
Public String stockid;                                                           
public List<cVehicle> vehicleList{get; set;}                                
public Boolean selectAllCheckbox {get; set;}                                 
public boolean errormsg=false;                                               
String userinput;                                                            
String userinp;                                                              

Public boolean displayboxes;

Public List<Vehicle__c> results = new List<Vehicle__c>();                                    
Public List<Vehicle__c> selectedContactsstep2 = new List<Vehicle__c>();            
/* End of Variable declarations */

/* Getter and setter methods for getting the user input from the UI */

public String getuserinput(){return userinput;}
public void setuserinput(String userinp){this.userinput=userinp;}
public String getuserinp(){return userinp;}
public void setuserinp(String userinp){this.userinp=userinp;}


/*End of Getter and Setter methods */



/* Method to Search the */
public List<Contact> contactsearch()
{
     errormsg=false;
     vehicleList= new List<cVehicle>();
     for(Vehicle__c c : [select Stock_Number__c,Registration_number__c,Availability__c from Vehicle__c where Stock_Number__c like :+'%'+userinput+'%' and Registration_number__c like :+'%'+userinp+'%'])
     {
         vehicleList.add(new cVehicle(c));
     }

 return null;
}
/* End of method */


public List<cVehicle> getresults()
{

 return vehicleList;

}


public class cVehicle
{
 public Vehicle__c con {get; set;}
 public Boolean selected {get; set;}

 public cVehicle(Vehicle__c c)
 {
     con = c;
     selected = false;
 }
}

/* Method to fetch the selected records and show booking page*/
public PageReference processSelected()
{

  List<Vehicle__c> selectedContacts = new List<Vehicle__c>();
  if (vehicleList!= null)
  {
     for(cVehicle cCon : getresults()){
        if(cCon.selected == true){
             selectedContacts.add(cCon.con);
         }
     }

     selectedContactsstep2=selectedContacts;
  }
  if (selectedcontacts.size()==0)
 {
     errormsg=true;
     return null;
 }
 else
 {
     errormsg=false;
     String id=System.currentpagereference().getparameters().get('id');
     String stocknum;
     for (Vehicle__c c: selectedcontacts){
         stocknum = c.Stock_Number__c;
     }
     Pagereference p=new Pagereference ('/apex/getDetails_booking?val='+stocknum);
    
     return p;
 }

}
public List<SelectOption> getItems()
{
 List<SelectOption> options = new List<SelectOption>();
 options.add(new SelectOption('YES','YES'));
 options.add(new SelectOption('NO','NO'));
 return options;
}


public boolean geterrormsg()
{
 return errormsg;
}


public Pagereference Cancel()
{
        Pagereference p =new Pagereference('/'+stockid);
        return p;
}
      

}