What is the purpose of the "religion" field on the Vietnam EVisa application?

According to Smartraveller, Vietnam cracks down on unapproved religious activities:

involvement in non-state sanctioned political or religious activities, including on-line activities – those suspected could be denied entry into Vietnam, detained, deported or prevented from departing Vietnam until authorities have completed investigations

My theory is that Christianity, or certain proselytising denominations, may be treated with suspicion, since some of them have a tendency to target nominally communist countries.

The conventional and probably correct response in backpacker forums is to put "No."

A wrong answer would be religions that at the time of your entry are actively challenging the authority of the Communist government; historically that's most notably been Buddhist and Catholic. From Wikipedia's Freedom of Religion in Vietnam:

The New Ordinance on Religion and Belief, which came into effect in November 2004, serves as the primary document governing religious practice. It reiterates citizens' rights to freedom of belief, freedom of religion, and freedom not to follow a religion, and it states that violations of these freedoms are prohibited. However, it advises that "abuse" of freedom of belief or religion "to undermine the country's peace, independence, and unity" is illegal and warns that religious activities must be suspended if they negatively affect the cultural traditions of the nation.

For instance,

The government requires all Buddhist monks to be approved by and work under the officially recognized Buddhist organization, the Vietnam Buddhist Sangha (VBS)...

Article 35 of Decree 22 requires government approval for foreign missionary groups to proselytize...

Government policy does not permit persons who belong to unofficial religious groups to speak publicly about their beliefs...

Contacts between some unregistered Protestant organizations and their foreign supporters are discouraged but occur regularly...

I'm going to guess this is probably your first visit to a Communist country? I've been to them from the USSR to North Korea, and I'm pretty sure there's never been a visa application that didn't ask for my religion, and when I put "Christian" I routinely get asked if I have any Bibles with me -- that was the only question I got asked entering North Korea last year, in fact!