I have to build a web application which uses Python, php and MongoDB.

Python - For offline database populating on my local home machine and then exporting db to VPS. Later I am planning to schedule this job using cron.
PHP - For web scripting.

The VPS I wish to buy supports Python and LAMP Stack but not mongoDB (myhosting.com LAMP stack VPS) by default. Now since mongoDB isn't supported by default, I would have to install mongoDB manually on VPS. So what I want to know is that, had it been my VPS would have supported mongoDB would I have benefitted in terms of performance and scalability.
Also can someone please suggest a VPS suitable in my case.
If the vps you are looking at restricts the packages you can install, and you need something that they prohibit, I would look for another vps.  Both rackspace and amazon a range of instances, and numerous supported os.  With either of them you choose your operating system and are free to install whatever you want.
Performance of MongoDB on VPS or cloud service not having mongoDB installed