Overtype/input for lookup field

The lookup is working fine currently but they have a few thousand records that may be added into the list

this is not 'optimal' neither from user experience point of view nor from performance one.

I'd recommend to classify such long list into several hierarchical categories and let the user to select first categories that restrict list of available options in lookup list

is there a way of doing the lookup with a text field

sure, you can define a lookup field from a text column. it will internally anyway represent the lookup value as KeyField;#ValueField (KeyField being item ID)

Due to the volume of records they obviously don't want to have to scroll through thousands of VINs

they can put focus on lookup control and start typing, it will incrementally search the list of option. if the list is sorted they should be able to get to the being looked up option relatively fast.

Is there a way they could overtype?

do you mean to provide a custom value, not (yet) available in the list?

nintex lookup control doesn't support this functionality.

, is there a way they could enter the VIN and have it validated against the Vin list? 

yes, you can build a validation rule based on a lookup() function.

formula might look like

length(lookup('VINList','VINListField',VINFormNamedControl,'ID',true))<1‍‍‍