The Hammers signed the midfielder from CSKA Moscow last summer for a fee of £25.7m and paid the first instalment when the deal was agreed.
West Ham are due to pay the next part of the payment to CSKA Moscow soon, but that is likely to be more complicated than previously assumed.
Sanctions imposed by the UK government on Russian businesses since the country invaded Ukraine last week will have an effect.
The east London club are not sure whether they will be allowed to carry out any business transactions with CSKA Moscow.
The sanctions imposed by the government could stop English clubs from doing business with Russian outfits.
It could mean that the next instalment for Vlasic’s transfer might be delayed or not allowed to go through.
The FA have stressed that the clubs will comply, but it is unclear how CSKA Moscow would react to any non-payment of the next instalment.
