This sort of thing:

I've just checked a few of my packages and noticed that I generally issue parameterised calls to stored procs, rather than parameterised DML. So I cannot confirm right now that...

Apart from splitting it into two queries, I can't see any way of simplifying what you have written:

DECLARE @max_rmp_vkey INT = (
           SELECT MAX(rmp_vkey)

Have you tried building the SQL in an expression and using that as your data source (ie, no parameters)?
You might have more success if you explicitly name your columns...