I'm trying to write a trigger on opportunities that when an opportunity is closed, it will update the invoice date field on a custom object. The records are related based on a unique field on the custom object that is in the description field on the opportunity. The field is called Work_order_id_c on the custom object. I don't know how to assign the CloseDate from the mapWOIds map to the vdDocs InvoiceDate using the WorkOrderID. Any help would be appreciated.

trigger UpdateVersaDocDocument on Opportunity (after insert, after update) {
	 if (Trigger.isInsert){
	 	map<String, Date> mapWOIds = new map<String, Date>();
	 	set<String> workOrderIds = new Set<String>();
	 	list<VersaDocDocuments__c> vdDocsUpdate = new list<VersaDocDocuments__c>();
		
		// loop through trigger and add WorkOrderID and Invoice date to map 	
        for (Opportunity o : trigger.new){
        	if (o.StageName == 'Closed/Won' && o.Type == 'Customization' && o.Description.startsWith('WorkOrderID:')) {
               	mapWOIds.put(o.Description.substringAfter(': '), o.CloseDate);  
               	workOrderIds.add(o.Description.substringAfter(': '));               	            	
        	}        	
        }
        // create a map a VersaDoc Documents
       map<String, VersaDocDocuments__c> vdDocs = new map<String, VersaDocDocuments__c>([Select Work_Order_ID__c, Id, Invoice_date__c from VersaDocDocuments__c Where Work_Order_ID__c in :workOrderIds]);

        // iterate over the list of versadoc Documents and assign the invoice date
       	for (String vd : vdDocs.keySet()) {
       		System.debug('in for loop vddocs.key is ' + vdDocs.get(vd));
       		vd.Invoice_Date__c = mapWOIds.get(vdDocs.get(vd));
        	// vdDocsUpdate.add(vd);
      		    		     	
        }  
           
	 }
}