The VIN is the whole number. Every RMV in the country recognizes all the charaters as the complete VIN. Otherwise there would be duplicates all over the place. Since 1981 the Fed GVMT has mandated a 17 digit VIN system. Please put that to bed. [/b]
Done

As was stated, the VIN will tell all. If you are going to 'backdate' your chassis, the VIN must not include a digit from the 'wrong' engine or from the 'wrong' year (if the engine were not produced at the same time) or signify any options or trim packages that which would prove you have violated the VIN rule.[/b]
Well, perhaps the VIN should tell all, but it really does not in many cases.

Many have debated here the 'intellegence' of this rule, but it still is a rule.[/b]
Agreed.

Having said all that, there still may be VIN's that don't tell you what you want to know.
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Yes, some give the same code letter for two or more engine types that are on different ITCS lines and/or classes.

VW VIN decoder
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That particular "decoder" has more errors than a beer soaked baseball game. Please don't use it as gospel. It is marginally accurate for VWs produced in Pennsylvania from 81-88, but way off for German, Mexican and Brazilian production. My bet is that it's also going to be incorrect for the 1987 Golf 1.8L engines (since it often takes VW an extra year to differentiate new engine offerings and that was the first year for the 16V GTI), but the only way to confirm is going to be to check several VINs from actual examples.