It is unclear, why someone has down voted, without leaving a comment...

When I look at the original bitcoin specs, I can see:" Added in Bitcoin Core 0.13.0 - The virtual transaction size. Differs from size for witness transactions". I tried to run through the page "https://bitcoin.org/en/release/v0.13.0", but there was no mentioning of virtual size. So it seems to be hidden in one of the many SegWit additions. On https://bitcoincore.org/en/segwit_wallet_dev/ I finally found this:

Transaction Fee Estimation

- Instead of transaction size, a new metric is defined, called “virtual size” (vsize)
- vsize of a transaction equals to 3 times of the size with original serialization, plus the size with new serialization, divide the result by 4 and round up to the next integer. For example, if a transaction is 200 bytes with new serialization, and becomes 99 bytes with marker, flag, and witness removed, the vsize is (99 * 3 + 200) / 4 = 125 with round up.
- vsize of a non-segwit transaction is simply its size
- Transaction fee should be estimated by comparing the vsize with other transactions, not the size.
- Developers should be careful not to make an off-by-4-times mistake in fee estimation.

Then I looked into

• blockchain.info
• block explorer.com
• live.blockcypher.com
• www.blockcypher.com
• www.blocktrail.com

just to find out, they don't provide this value...