The funds are currently are currently top two performers in North American Smaller Companies sector on a three-year basis.
Vanguard said the move was designed to protect the interests of current shareholders and also safeguard the ability of the small-cap managers to effectively run the respective funds.
Existing shareholders will be permitted to continue making additional share purchases without limitations in existing accounts.
It has seen assets increase from $25 million at the end of 2012 to $160 million as at 31 December 2013.
This fund has returned 84.9% over the past three years, while its benchmark, the Russell 2000 Growth TR, rose 57.55%. It has also featured prominently in Citywire analysis into consistent performance in the US small-cap sector.
Meanwhile, the Vanguard US Opportunities fund, which has returned 72.4% over the same time frame and against the same index, has also seen a considerable growth in assets.
This fund, which is managed on mandate by the PRIMECAP Management Company, has risen from $850 million at the end of 2012 to $1.8 billion one year later.
Commenting on the closures, Thomas Rampulla, managing director for Vanguard in Europe, said: ‘Vanguard is proactively taking steps to reduce cash flow into these funds in order to help preserve the portfolio managers’ ability to effectively manage the funds.
‘Our commitment is to protect the interests of the fund’s current shareholders and, when necessary, we take pre-emptive action to restrict cash inflows to maintain fund assets at reasonable levels.’
Vanguard has said it will monitor the cash flow of the two funds with a view to reopening or further closures for the funds.