Whether it is prosaic wisdom such as “sell in May and go away” or more tactical advice such as buying consumer discretionary stocks early in the first quarter or energy names in February, there are a plenty of seasonal trends investors can study.

One that does not get much attention is the tendency of the Market Vectors Vietnam ETF (NYSEArca: VNM) to slide in the second quarter. After falling almost 3% on heavy volume Monday, the lone Vietnam ETF is lower by nearly 6% this month. That could prove to be an ominous sign for an ETF that has never posted a second-quarter gain.

VNM debuted in August 2009 and its best second-quarter performance was a 3.1% loss in 2010. The ETF’s average second-quarter loss over the past four years is 7.6%, according to ETF Replay data.

Different factors have afflicted in VNM in various second quarters. In years past, the ETF has been hit by Vietnam’s inability to contain inflation, dong devaluations and banking scandals involving the country’s equivalent of Russian oligarchs. [Vietnam ETF Rallies on Impressive Data]

To be fair, those issues appear to be in the past and that explains why even with Monday’s loss, VNM is still up 8.6%. That is well ahead of the returns offered by broader emerging markets ETFs, but Vietnam is classified as a frontier market, making the more relevant barometer the iShares MSCI Frontier 100 ETF (NYSEArca: FM), an ETF that VNM is trailing this year. [Other Frontier ETFs Gain Supporters]

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