The question is rather simple but the answer is not.
Lower thermal (heating) value of that mixture of gaseous hydrocarbons and air depends on amount and nature of hydrocarbons. In other words depending on temperature and pressure in a tank and certainly type of the oil plays a role in it (content of readily volatile light end hydrocarbons in the oil).
Therefore, the proper answer may vary but I believe that it can’t be higher than half the natural gas thermal value. My wild assumption is that vapour can’t occupy more than half the volume (even that looks too much) and its composition is close to the natural gas composition.
A number of modern tankers use the VOCs as fuel. I would expect that there is practical data available from those sources. Try the major web sites such as Wartsila, Man B&W, Caterpillar.
Otherwise you may be faced with measuring this.
