VPP proponents claim that the NIV is literally “corrupted” or “demonic”

The criticism exists, but it helps to distinguish legitimate translation criticism from claims that the NIV is literally “corrupted” or “demonic.”

Why some Christians strongly oppose the NIV

1. The NIV differs from the KJV in many verses.
The biggest source of controversy is not that NIV translators secretly removed Christianity, but that modern translations use a different approach to the ancient Greek manuscripts.

The KJV was based largely on the Textus Receptus, while the NIV's New Testament draws on a much broader body of Greek manuscript evidence, including manuscripts discovered or better understood after the KJV was produced.

Consequently, some verses or phrases that appear in the KJV are absent from the NIV or placed in footnotes. Examples frequently discussed include parts of Matthew 18:11, Mark 16:9–20, John 7:53–8:11, and Acts 8:37.

For KJV-only Christians, this can look like someone has removed Scripture. For modern textual critics, it is generally understood as an attempt to determine what the earliest recoverable text said.

2. Some NIV wording is genuinely controversial.
The 2011 NIV, for example, uses gender-inclusive wording in some contexts where translators believe the underlying Greek or Hebrew refers to people generally rather than specifically to males.

Critics argue that this can sometimes move beyond translation into interpretation. Supporters argue that English itself has changed and that translating an ancient generic masculine expression literally can sometimes communicate the wrong meaning to modern readers.

That is a real translation debate. It doesn't require the conclusion that the translators were demonically motivated.

3. The “Lucifer” controversy is another example.
Critics sometimes point to Isaiah 14:12 and the NIV's rendering “morning star” rather than the traditional KJV “Lucifer.”

But the Hebrew expression there is hêlēl ben-šaḥar, commonly understood along the lines of “shining one, son of dawn.” The Latin Vulgate's lucifer (“light-bearer/morning star”) eventually became the English proper name “Lucifer.”

So the difference is primarily a textual and translation-history issue, not evidence that the NIV is worshipping Satan.

What about calling it “demonic”?

That's a theological accusation rather than something that can be established from manuscript or linguistic evidence.

It's possible to believe:

“I think the NIV makes too many questionable translation choices.”

That's a legitimate position.

It's quite different from:

“The NIV was produced by demons” or “the NIV is a corrupted Bible.”

The second claim requires evidence of intentional corruption or supernatural involvement, and the existence of differences between the NIV and KJV doesn't establish either.

There are also plenty of serious scholarly criticisms of the NIV that don't depend on sensational claims. Someone can prefer the KJV, NKJV, NASB, ESV, or another translation because they believe its textual basis or translation philosophy is preferable.

At the same time, NIV translators are not simply making arbitrary changes to Scripture. The translation committee explains its methodology, textual decisions, and revision process, and the NIV has been produced through decades of engagement with Hebrew, Aramaic, Greek, textual criticism, and English-language scholarship.

So I'd put it this way:

“The NIV should be critically evaluated” — yes.
“The NIV has passages whose translation is debatable” — yes.
“The NIV differs significantly from the KJV in some passages” — yes.
“Therefore the NIV is corrupted or demonic” — that conclusion does not follow from those facts.